diff --git a/docs/source/1.0/spec/aws/aws-ec2-query-protocol.rst b/docs/source/1.0/spec/aws/aws-ec2-query-protocol.rst
index 52de2616157..8002f8320fb 100644
--- a/docs/source/1.0/spec/aws/aws-ec2-query-protocol.rst
+++ b/docs/source/1.0/spec/aws/aws-ec2-query-protocol.rst
@@ -23,7 +23,9 @@ Summary
     OR in a ``x-form-url-encoded`` body and responses in XML documents. This
     protocol is an Amazon EC2-specific extension of the ``awsQuery`` protocol.
 Trait selector
-    ``service``
+    ``service [trait|xmlNamespace]``
+
+    *Service shapes with the xmlNamespace trait*
 Value type
     Annotation trait.
 
diff --git a/docs/source/1.0/spec/aws/aws-query-protocol.rst b/docs/source/1.0/spec/aws/aws-query-protocol.rst
index d2dbe30761c..4689c052d4d 100644
--- a/docs/source/1.0/spec/aws/aws-query-protocol.rst
+++ b/docs/source/1.0/spec/aws/aws-query-protocol.rst
@@ -22,7 +22,9 @@ Summary
     Adds support for an HTTP protocol that sends requests in the query
     string and responses in XML documents.
 Trait selector
-    ``service``
+    ``service [trait|xmlNamespace]``
+
+    *Service shapes with the xmlNamespace trait*
 Value type
     Annotation trait.
 See
diff --git a/docs/source/1.0/spec/core/xml-traits.rst b/docs/source/1.0/spec/core/xml-traits.rst
index 1475b7b179f..6c5ce50fc02 100644
--- a/docs/source/1.0/spec/core/xml-traits.rst
+++ b/docs/source/1.0/spec/core/xml-traits.rst
@@ -937,7 +937,9 @@ The XML serialization is:
 Summary
     Adds an `XML namespace`_ to an XML element.
 Trait selector
-    ``*``
+    ``:is(service, member, simpleType, collection, map, structure, union)``
+
+    *Service, simple types, list, map, set, structure, or union*
 Value type
     ``structure``
 Conflicts with
diff --git a/smithy-aws-protocol-tests/model/awsQuery/main.smithy b/smithy-aws-protocol-tests/model/awsQuery/main.smithy
index 7046a0742fc..0a430261fbe 100644
--- a/smithy-aws-protocol-tests/model/awsQuery/main.smithy
+++ b/smithy-aws-protocol-tests/model/awsQuery/main.smithy
@@ -35,6 +35,7 @@ service AwsQuery {
         XmlMapsXmlName,
         FlattenedXmlMap,
         FlattenedXmlMapWithXmlName,
+        FlattenedXmlMapWithXmlNamespace,
         XmlEmptyMaps,
 
         // Output XML list tests
diff --git a/smithy-aws-protocol-tests/model/awsQuery/xml-lists.smithy b/smithy-aws-protocol-tests/model/awsQuery/xml-lists.smithy
index e91e49183d2..970941f6de2 100644
--- a/smithy-aws-protocol-tests/model/awsQuery/xml-lists.smithy
+++ b/smithy-aws-protocol-tests/model/awsQuery/xml-lists.smithy
@@ -79,6 +79,10 @@ apply XmlLists @httpResponseTests([
                       <flattenedList>bye</flattenedList>
                       <customName>yep</customName>
                       <customName>nope</customName>
+                      <flattenedListWithMemberNamespace xmlns="https://xml-member.example.com">a</flattenedListWithMemberNamespace>
+                      <flattenedListWithMemberNamespace xmlns="https://xml-member.example.com">b</flattenedListWithMemberNamespace>
+                      <flattenedListWithNamespace>a</flattenedListWithNamespace>
+                      <flattenedListWithNamespace>b</flattenedListWithNamespace>
                       <myStructureList>
                           <item>
                               <value>1</value>
@@ -107,6 +111,8 @@ apply XmlLists @httpResponseTests([
             renamedListMembers: ["foo", "bar"],
             flattenedList: ["hi", "bye"],
             flattenedList2: ["yep", "nope"],
+            flattenedListWithMemberNamespace: ["a", "b"],
+            flattenedListWithNamespace: ["a", "b"],
             structureList: [
                 {
                     a: "1",
@@ -180,6 +186,17 @@ structure XmlListsOutput {
     // serializing flattened lists in structures.
     flattenedList2: RenamedListMembers,
 
+    // The XML namespace of the flattened list's member is used, and
+    // list's XML namespace is disregarded.
+    @xmlFlattened
+    flattenedListWithMemberNamespace: ListWithMemberNamespace,
+
+    // Again, the XML namespace of the flattened list is ignored.
+    // The namespace of the member is used, which is empty, so
+    // no xmlns attribute appears on the serialized XML.
+    @xmlFlattened
+    flattenedListWithNamespace: ListWithNamespace,
+
     @xmlName("myStructureList")
     structureList: StructureList
 }
@@ -201,3 +218,14 @@ structure StructureListMember {
     @xmlName("other")
     b: String,
 }
+
+@xmlNamespace(uri: "https://xml-list.example.com")
+list ListWithMemberNamespace {
+    @xmlNamespace(uri: "https://xml-member.example.com")
+    member: String,
+}
+
+@xmlNamespace(uri: "https://xml-list.example.com")
+list ListWithNamespace {
+    member: String,
+}
diff --git a/smithy-aws-protocol-tests/model/awsQuery/xml-maps.smithy b/smithy-aws-protocol-tests/model/awsQuery/xml-maps.smithy
index 0d3ff2006a6..7c686615d19 100644
--- a/smithy-aws-protocol-tests/model/awsQuery/xml-maps.smithy
+++ b/smithy-aws-protocol-tests/model/awsQuery/xml-maps.smithy
@@ -269,3 +269,57 @@ map FlattenedXmlMapWithXmlNameOutputMap {
     @xmlName("V")
     value: String,
 }
+
+/// Flattened maps with @xmlNamespace and @xmlName
+operation FlattenedXmlMapWithXmlNamespace {
+    output: FlattenedXmlMapWithXmlNamespaceOutput
+}
+
+apply FlattenedXmlMapWithXmlNamespace @httpResponseTests([
+    {
+        id: "QueryQueryFlattenedXmlMapWithXmlNamespace",
+        documentation: "Serializes flattened XML maps in responses that have xmlNamespace and xmlName on members",
+        protocol: awsQuery,
+        code: 200,
+        body: """
+              <FlattenedXmlMapWithXmlNamespaceResponse xmlns="https://example.com/">
+                  <FlattenedXmlMapWithXmlNamespaceResult>
+                      <KVP xmlns="https://the-member.example.com">
+                          <K xmlns="https://the-key.example.com">a</K>
+                          <V xmlns="https://the-value.example.com">A</V>
+                      </KVP>
+                      <KVP xmlns="https://the-member.example.com">
+                          <K xmlns="https://the-key.example.com">b</K>
+                          <V xmlns="https://the-value.example.com">B</V>
+                      </KVP>
+                  </FlattenedXmlMapWithXmlNamespaceResult>
+              </FlattenedXmlMapWithXmlNamespaceResponse>""",
+        bodyMediaType: "application/xml",
+        headers: {
+            "Content-Type": "text/xml"
+        },
+        params: {
+            myMap: {
+                a: "A",
+                b: "B",
+            }
+        }
+    }
+])
+
+structure FlattenedXmlMapWithXmlNamespaceOutput {
+    @xmlFlattened
+    @xmlName("KVP")
+    @xmlNamespace(uri: "https://the-member.example.com")
+    myMap: FlattenedXmlMapWithXmlNamespaceOutputMap,
+}
+
+map FlattenedXmlMapWithXmlNamespaceOutputMap {
+    @xmlName("K")
+    @xmlNamespace(uri: "https://the-key.example.com")
+    key: String,
+
+    @xmlName("V")
+    @xmlNamespace(uri: "https://the-value.example.com")
+    value: String,
+}
diff --git a/smithy-aws-protocol-tests/model/ec2Query/xml-lists.smithy b/smithy-aws-protocol-tests/model/ec2Query/xml-lists.smithy
index 1ea6f16dfd8..0e9beab5e94 100644
--- a/smithy-aws-protocol-tests/model/ec2Query/xml-lists.smithy
+++ b/smithy-aws-protocol-tests/model/ec2Query/xml-lists.smithy
@@ -23,7 +23,8 @@ use smithy.test#httpResponseTests
 /// 4. XML lists with @xmlName on its members
 /// 5. Flattened XML lists.
 /// 6. Flattened XML lists with @xmlName.
-/// 7. Lists of structures.
+/// 7. Flattened XML lists with @xmlNamespace.
+/// 8. Lists of structures.
 operation XmlLists {
     output: XmlListsOutput
 }
@@ -78,6 +79,10 @@ apply XmlLists @httpResponseTests([
                   <flattenedList>bye</flattenedList>
                   <customName>yep</customName>
                   <customName>nope</customName>
+                  <flattenedListWithMemberNamespace xmlns="https://xml-member.example.com">a</flattenedListWithMemberNamespace>
+                  <flattenedListWithMemberNamespace xmlns="https://xml-member.example.com">b</flattenedListWithMemberNamespace>
+                  <flattenedListWithNamespace>a</flattenedListWithNamespace>
+                  <flattenedListWithNamespace>b</flattenedListWithNamespace>
                   <myStructureList>
                       <item>
                           <value>1</value>
@@ -106,6 +111,8 @@ apply XmlLists @httpResponseTests([
             renamedListMembers: ["foo", "bar"],
             flattenedList: ["hi", "bye"],
             flattenedList2: ["yep", "nope"],
+            flattenedListWithMemberNamespace: ["a", "b"],
+            flattenedListWithNamespace: ["a", "b"],
             structureList: [
                 {
                     a: "1",
@@ -177,6 +184,17 @@ structure XmlListsOutput {
     // serializing flattened lists in structures.
     flattenedList2: RenamedListMembers,
 
+    // The XML namespace of the flattened list's member is used, and
+    // list's XML namespace is disregarded.
+    @xmlFlattened
+    flattenedListWithMemberNamespace: ListWithMemberNamespace,
+
+    // Again, the XML namespace of the flattened list is ignored.
+    // The namespace of the member is used, which is empty, so
+    // no xmlns attribute appears on the serialized XML.
+    @xmlFlattened
+    flattenedListWithNamespace: ListWithNamespace,
+
     @xmlName("myStructureList")
     structureList: StructureList
 }
@@ -198,3 +216,14 @@ structure StructureListMember {
     @xmlName("other")
     b: String,
 }
+
+@xmlNamespace(uri: "https://xml-list.example.com")
+list ListWithMemberNamespace {
+    @xmlNamespace(uri: "https://xml-member.example.com")
+    member: String,
+}
+
+@xmlNamespace(uri: "https://xml-list.example.com")
+list ListWithNamespace {
+    member: String,
+}
diff --git a/smithy-aws-protocol-tests/model/restXml/document-lists.smithy b/smithy-aws-protocol-tests/model/restXml/document-lists.smithy
index 7743210874d..288cf4c4eb3 100644
--- a/smithy-aws-protocol-tests/model/restXml/document-lists.smithy
+++ b/smithy-aws-protocol-tests/model/restXml/document-lists.smithy
@@ -26,7 +26,8 @@ use smithy.test#httpResponseTests
 /// 4. XML lists with @xmlName on its members
 /// 5. Flattened XML lists.
 /// 6. Flattened XML lists with @xmlName.
-/// 7. Lists of structures.
+/// 7. Flattened XML lists with @xmlNamespace.
+/// 8. Lists of structures.
 @idempotent
 @http(uri: "/XmlLists", method: "PUT")
 operation XmlLists {
@@ -176,6 +177,10 @@ apply XmlLists @httpResponseTests([
                   <flattenedList>bye</flattenedList>
                   <customName>yep</customName>
                   <customName>nope</customName>
+                  <flattenedListWithMemberNamespace xmlns="https://xml-member.example.com">a</flattenedListWithMemberNamespace>
+                  <flattenedListWithMemberNamespace xmlns="https://xml-member.example.com">b</flattenedListWithMemberNamespace>
+                  <flattenedListWithNamespace>a</flattenedListWithNamespace>
+                  <flattenedListWithNamespace>b</flattenedListWithNamespace>
                   <myStructureList>
                       <item>
                           <value>1</value>
@@ -203,6 +208,8 @@ apply XmlLists @httpResponseTests([
             renamedListMembers: ["foo", "bar"],
             flattenedList: ["hi", "bye"],
             flattenedList2: ["yep", "nope"],
+            flattenedListWithMemberNamespace: ["a", "b"],
+            flattenedListWithNamespace: ["a", "b"],
             structureList: [
                 {
                     a: "1",
@@ -300,6 +307,17 @@ structure XmlListsInputOutput {
     // serializing flattened lists in structures.
     flattenedList2: RenamedListMembers,
 
+    // The XML namespace of the flattened list's member is used, and
+    // list's XML namespace is disregarded.
+    @xmlFlattened
+    flattenedListWithMemberNamespace: ListWithMemberNamespace,
+
+    // Again, the XML namespace of the flattened list is ignored.
+    // The namespace of the member is used, which is empty, so
+    // no xmlns attribute appears on the serialized XML.
+    @xmlFlattened
+    flattenedListWithNamespace: ListWithNamespace,
+
     @xmlName("myStructureList")
     structureList: StructureList
 }
@@ -321,3 +339,14 @@ structure StructureListMember {
     @xmlName("other")
     b: String,
 }
+
+@xmlNamespace(uri: "https://xml-list.example.com")
+list ListWithMemberNamespace {
+    @xmlNamespace(uri: "https://xml-member.example.com")
+    member: String,
+}
+
+@xmlNamespace(uri: "https://xml-list.example.com")
+list ListWithNamespace {
+    member: String,
+}
diff --git a/smithy-aws-protocol-tests/model/restXml/document-maps.smithy b/smithy-aws-protocol-tests/model/restXml/document-maps.smithy
index d6c240f1e0e..ec078390a65 100644
--- a/smithy-aws-protocol-tests/model/restXml/document-maps.smithy
+++ b/smithy-aws-protocol-tests/model/restXml/document-maps.smithy
@@ -433,3 +433,56 @@ map FlattenedXmlMapWithXmlNameInputOutputMap {
     @xmlName("V")
     value: String,
 }
+
+/// Flattened maps with @xmlNamespace and @xmlName
+@http(uri: "/FlattenedXmlMapWithXmlNamespace", method: "POST")
+operation FlattenedXmlMapWithXmlNamespace {
+    output: FlattenedXmlMapWithXmlNamespaceOutput
+}
+
+apply FlattenedXmlMapWithXmlNamespace @httpResponseTests([
+    {
+        id: "RestXmlFlattenedXmlMapWithXmlNamespace",
+        documentation: "Serializes flattened XML maps in responses that have xmlNamespace and xmlName on members",
+        protocol: restXml,
+        code: 200,
+        body: """
+              <FlattenedXmlMapWithXmlNamespaceOutput>
+                  <KVP xmlns="https://the-member.example.com">
+                      <K xmlns="https://the-key.example.com">a</K>
+                      <V xmlns="https://the-value.example.com">A</V>
+                  </KVP>
+                  <KVP xmlns="https://the-member.example.com">
+                      <K xmlns="https://the-key.example.com">b</K>
+                      <V xmlns="https://the-value.example.com">B</V>
+                  </KVP>
+              </FlattenedXmlMapWithXmlNamespaceOutput>""",
+        bodyMediaType: "application/xml",
+        headers: {
+            "Content-Type": "application/xml"
+        },
+        params: {
+            myMap: {
+                a: "A",
+                b: "B",
+            }
+        }
+    }
+])
+
+structure FlattenedXmlMapWithXmlNamespaceOutput {
+    @xmlFlattened
+    @xmlName("KVP")
+    @xmlNamespace(uri: "https://the-member.example.com")
+    myMap: FlattenedXmlMapWithXmlNamespaceOutputMap,
+}
+
+map FlattenedXmlMapWithXmlNamespaceOutputMap {
+    @xmlName("K")
+    @xmlNamespace(uri: "https://the-key.example.com")
+    key: String,
+
+    @xmlName("V")
+    @xmlNamespace(uri: "https://the-value.example.com")
+    value: String,
+}
diff --git a/smithy-aws-protocol-tests/model/restXml/main.smithy b/smithy-aws-protocol-tests/model/restXml/main.smithy
index 51ec85e57b1..1829415ce69 100644
--- a/smithy-aws-protocol-tests/model/restXml/main.smithy
+++ b/smithy-aws-protocol-tests/model/restXml/main.smithy
@@ -71,6 +71,7 @@ service RestXml {
         XmlMapsXmlName,
         FlattenedXmlMap,
         FlattenedXmlMapWithXmlName,
+        FlattenedXmlMapWithXmlNamespace,
 
         // @xmlAttribute tests
         XmlAttributes,
diff --git a/smithy-aws-protocol-tests/model/restXmlWithNamespace/main.smithy b/smithy-aws-protocol-tests/model/restXmlWithNamespace/main.smithy
new file mode 100644
index 00000000000..0cb5c062bcd
--- /dev/null
+++ b/smithy-aws-protocol-tests/model/restXmlWithNamespace/main.smithy
@@ -0,0 +1,131 @@
+$version: "1.0"
+
+namespace aws.protocoltests.restxml.xmlns
+
+use aws.api#service
+use aws.protocols#restXml
+use smithy.test#httpRequestTests
+use smithy.test#httpResponseTests
+
+/// A REST XML service that sends XML requests and responses.
+///
+/// This service and test case is complementary to the test cases
+/// in the `restXml` directory, but the service under test here has
+/// the `xmlNamespace` trait applied to it.
+///
+/// See https://github.com/awslabs/smithy/issues/616
+@service(sdkId: "Rest Xml Protocol Namespace")
+@xmlNamespace(uri: "https://example.com")
+@restXml
+service RestXmlWithNamespace {
+    version: "2019-12-16",
+    operations: [SimpleScalarProperties]
+}
+
+// This example serializes simple scalar types in the top level XML document.
+// Note that headers are not serialized in the payload.
+//
+// This is a partial copy of aws.protocoltests.restxml#SimpleScalarProperties,
+// but only includes enough test cases to ensure a namespace is serialized.
+@idempotent
+@http(uri: "/SimpleScalarProperties", method: "PUT")
+operation SimpleScalarProperties {
+    input: SimpleScalarPropertiesInputOutput,
+    output: SimpleScalarPropertiesInputOutput
+}
+
+apply SimpleScalarProperties @httpRequestTests([
+    {
+        id: "XmlNamespaceSimpleScalarProperties",
+        documentation: "Serializes simple scalar properties",
+        protocol: restXml,
+        method: "PUT",
+        uri: "/SimpleScalarProperties",
+        body: """
+              <SimpleScalarPropertiesInputOutput xmlns="https://example.com">
+                  <stringValue>string</stringValue>
+                  <trueBooleanValue>true</trueBooleanValue>
+                  <falseBooleanValue>false</falseBooleanValue>
+                  <byteValue>1</byteValue>
+                  <shortValue>2</shortValue>
+                  <integerValue>3</integerValue>
+                  <longValue>4</longValue>
+                  <floatValue>5.5</floatValue>
+                  <DoubleDribble>6.5</DoubleDribble>
+              </SimpleScalarPropertiesInputOutput>
+              """,
+        bodyMediaType: "application/xml",
+        headers: {
+            "Content-Type": "application/xml",
+            "X-Foo": "Foo",
+        },
+        params: {
+            foo: "Foo",
+            stringValue: "string",
+            trueBooleanValue: true,
+            falseBooleanValue: false,
+            byteValue: 1,
+            shortValue: 2,
+            integerValue: 3,
+            longValue: 4,
+            floatValue: 5.5,
+            doubleValue: 6.5,
+        }
+    }
+])
+
+apply SimpleScalarProperties @httpResponseTests([
+    {
+        id: "XmlNamespaceSimpleScalarProperties",
+        documentation: "Serializes simple scalar properties",
+        protocol: restXml,
+        code: 200,
+        body: """
+              <SimpleScalarPropertiesInputOutput xmlns="https://example.com">
+                  <stringValue>string</stringValue>
+                  <trueBooleanValue>true</trueBooleanValue>
+                  <falseBooleanValue>false</falseBooleanValue>
+                  <byteValue>1</byteValue>
+                  <shortValue>2</shortValue>
+                  <integerValue>3</integerValue>
+                  <longValue>4</longValue>
+                  <floatValue>5.5</floatValue>
+                  <DoubleDribble>6.5</DoubleDribble>
+              </SimpleScalarPropertiesInputOutput>
+              """,
+        bodyMediaType: "application/xml",
+        headers: {
+            "Content-Type": "application/xml",
+            "X-Foo": "Foo",
+        },
+        params: {
+            foo: "Foo",
+            stringValue: "string",
+            trueBooleanValue: true,
+            falseBooleanValue: false,
+            byteValue: 1,
+            shortValue: 2,
+            integerValue: 3,
+            longValue: 4,
+            floatValue: 5.5,
+            doubleValue: 6.5,
+        }
+    }
+])
+
+structure SimpleScalarPropertiesInputOutput {
+    @httpHeader("X-Foo")
+    foo: String,
+
+    stringValue: String,
+    trueBooleanValue: Boolean,
+    falseBooleanValue: Boolean,
+    byteValue: Byte,
+    shortValue: Short,
+    integerValue: Integer,
+    longValue: Long,
+    floatValue: Float,
+
+    @xmlName("DoubleDribble")
+    doubleValue: Double,
+}
diff --git a/smithy-aws-traits/src/main/resources/META-INF/smithy/aws.protocols.json b/smithy-aws-traits/src/main/resources/META-INF/smithy/aws.protocols.json
index 9669251129d..0a1506679cf 100644
--- a/smithy-aws-traits/src/main/resources/META-INF/smithy/aws.protocols.json
+++ b/smithy-aws-traits/src/main/resources/META-INF/smithy/aws.protocols.json
@@ -124,7 +124,7 @@
             "type": "structure",
             "traits": {
                 "smithy.api#trait": {
-                    "selector": "service"
+                    "selector": "service [trait|xmlNamespace]"
                 },
                 "smithy.api#protocolDefinition": {
                     "noInlineDocumentSupport": true,
@@ -143,7 +143,7 @@
             "type": "structure",
             "traits": {
                 "smithy.api#trait": {
-                    "selector": "service"
+                    "selector": "service [trait|xmlNamespace]"
                 },
                 "smithy.api#protocolDefinition": {
                     "noInlineDocumentSupport": true,
diff --git a/smithy-aws-traits/src/test/resources/software/amazon/smithy/aws/traits/errorfiles/protocols/xml-protocols-do-not-support-documents.smithy b/smithy-aws-traits/src/test/resources/software/amazon/smithy/aws/traits/errorfiles/protocols/xml-protocols-do-not-support-documents.smithy
index 2852cba7e2e..3d1b24ad37f 100644
--- a/smithy-aws-traits/src/test/resources/software/amazon/smithy/aws/traits/errorfiles/protocols/xml-protocols-do-not-support-documents.smithy
+++ b/smithy-aws-traits/src/test/resources/software/amazon/smithy/aws/traits/errorfiles/protocols/xml-protocols-do-not-support-documents.smithy
@@ -7,6 +7,7 @@ use aws.protocols#awsQuery
 
 @awsQuery
 @suppress(["DeprecatedTrait"]) // ignore the fact that the awsQuery trait is deprecated
+@xmlNamespace(uri: "https://example.com")
 service InvalidExample {
     version: "2020-06-15",
     operations: [Operation1]
diff --git a/smithy-model/src/main/resources/software/amazon/smithy/model/loader/prelude.smithy b/smithy-model/src/main/resources/software/amazon/smithy/model/loader/prelude.smithy
index b48215dec1e..66182540152 100644
--- a/smithy-model/src/main/resources/software/amazon/smithy/model/loader/prelude.smithy
+++ b/smithy-model/src/main/resources/software/amazon/smithy/model/loader/prelude.smithy
@@ -299,7 +299,8 @@ structure xmlFlattened {}
 string xmlName
 
 /// Adds an xmlns namespace definition URI to an XML element.
-@trait(conflicts: [xmlAttribute])
+@trait(selector: ":is(service, member, simpleType, collection, map, structure, union)",
+       conflicts: [xmlAttribute])
 @tags(["diff.error.const"])
 structure xmlNamespace {
     /// The namespace URI for scoping this XML element.