diff --git a/docs/source-1.0/spec/core/model.rst b/docs/source-1.0/spec/core/model.rst
index d184f61da76..1d7f33fa147 100644
--- a/docs/source-1.0/spec/core/model.rst
+++ b/docs/source-1.0/spec/core/model.rst
@@ -244,7 +244,7 @@ Shape IDs are formally defined by the following ABNF:
     AbsoluteRootShapeId  :`Namespace` "#" `Identifier`
     Namespace            :`Identifier` *("." `Identifier`)
     Identifier           :`IdentifierStart` *`IdentifierChars`
-    IdentifierStart      :*"_" ALPHA
+    IdentifierStart      :(1*"_" (ALPHA / DIGIT)) / ALPHA
     IdentifierChars      :ALPHA / DIGIT / "_"
     ShapeIdMember        :"$" `Identifier`
 
diff --git a/docs/source-2.0/spec/model.rst b/docs/source-2.0/spec/model.rst
index af962e4a4f8..e0e07b803f5 100644
--- a/docs/source-2.0/spec/model.rst
+++ b/docs/source-2.0/spec/model.rst
@@ -550,7 +550,7 @@ Shape IDs are formally defined by the following ABNF:
     AbsoluteRootShapeId  :`Namespace` "#" `Identifier`
     Namespace            :`Identifier` *("." `Identifier`)
     Identifier           :`IdentifierStart` *`IdentifierChars`
-    IdentifierStart      :*"_" ALPHA
+    IdentifierStart      :(1*"_" (ALPHA / DIGIT)) / ALPHA
     IdentifierChars      :ALPHA / DIGIT / "_"
     ShapeIdMember        :"$" `Identifier`
 
diff --git a/smithy-model/src/main/java/software/amazon/smithy/model/shapes/ShapeId.java b/smithy-model/src/main/java/software/amazon/smithy/model/shapes/ShapeId.java
index de421459cd7..dd2ba06ffde 100644
--- a/smithy-model/src/main/java/software/amazon/smithy/model/shapes/ShapeId.java
+++ b/smithy-model/src/main/java/software/amazon/smithy/model/shapes/ShapeId.java
@@ -131,13 +131,17 @@ private static int parseIdentifier(CharSequence identifier, int offset) {
             return -1;
         }
 
-        // Parse the required identifier_start production.
+        // Parse the required IdentifierStart production.
         char startingChar = identifier.charAt(offset);
         if (startingChar == '_') {
-            while (identifier.charAt(offset) == '_') {
+            while (offset < identifier.length() && identifier.charAt(offset) == '_') {
                 offset++;
             }
-            if (!ParserUtils.isValidIdentifierCharacter(identifier.charAt(offset))) {
+            if (offset == identifier.length()) {
+                return -1;
+            }
+            char current = identifier.charAt(offset);
+            if (!ParserUtils.isAlphabetic(current) && !ParserUtils.isDigit(current)) {
                 return -1;
             }
             offset++;
@@ -145,7 +149,7 @@ private static int parseIdentifier(CharSequence identifier, int offset) {
             return -1;
         }
 
-        // Parse the optional identifier_chars production.
+        // Parse the optional IdentifierChars production.
         while (offset < identifier.length()) {
             if (!ParserUtils.isValidIdentifierCharacter(identifier.charAt(offset))) {
                 // Return the position of the character that stops the identifier.
diff --git a/smithy-model/src/test/java/software/amazon/smithy/model/shapes/ShapeIdTest.java b/smithy-model/src/test/java/software/amazon/smithy/model/shapes/ShapeIdTest.java
index 15a4d68d9f4..fa81cadebc4 100644
--- a/smithy-model/src/test/java/software/amazon/smithy/model/shapes/ShapeIdTest.java
+++ b/smithy-model/src/test/java/software/amazon/smithy/model/shapes/ShapeIdTest.java
@@ -136,6 +136,57 @@ public void checksIfValidNamespaceName() {
         assertTrue(ShapeId.isValidNamespace("f.b1.c_d_1234.e"));
     }
 
+    @Test
+    public void checksIfValidNamespaceNameIdentifierOnly() {
+        // Empty String
+        assertFalse(ShapeId.isValidNamespace(""));
+
+        // Underscore Only
+        assertFalse(ShapeId.isValidNamespace("_"));
+        assertFalse(ShapeId.isValidNamespace("__"));
+
+        // Starts with DIGIT
+        assertFalse(ShapeId.isValidNamespace("1"));
+        assertFalse(ShapeId.isValidNamespace("1_"));
+        assertFalse(ShapeId.isValidNamespace("1a"));
+        assertFalse(ShapeId.isValidNamespace("12"));
+
+        // IdentifierStart: 1*"_" (ALPHA / DIGIT)
+        assertTrue(ShapeId.isValidNamespace("_a"));
+        assertTrue(ShapeId.isValidNamespace("_1"));
+        assertTrue(ShapeId.isValidNamespace("__a"));
+        assertTrue(ShapeId.isValidNamespace("__1"));
+        assertFalse(ShapeId.isValidNamespace("__#"));
+
+        // IdentifierStart: ALPHA
+        assertTrue(ShapeId.isValidNamespace("a"));
+        assertFalse(ShapeId.isValidNamespace("#"));
+
+        // Identifier: 1*"_" (ALPHA / DIGIT) *`IdentifierChars`
+        assertTrue(ShapeId.isValidNamespace("_ab"));
+        assertTrue(ShapeId.isValidNamespace("_a1"));
+        assertTrue(ShapeId.isValidNamespace("_a_"));
+        assertFalse(ShapeId.isValidNamespace("_a#"));
+        assertTrue(ShapeId.isValidNamespace("_1a"));
+        assertTrue(ShapeId.isValidNamespace("_12"));
+        assertTrue(ShapeId.isValidNamespace("_1_"));
+        assertFalse(ShapeId.isValidNamespace("_1#"));
+        assertTrue(ShapeId.isValidNamespace("__ab"));
+        assertTrue(ShapeId.isValidNamespace("__a1"));
+        assertTrue(ShapeId.isValidNamespace("__a_"));
+        assertFalse(ShapeId.isValidNamespace("__a#"));
+        assertTrue(ShapeId.isValidNamespace("__1a"));
+        assertTrue(ShapeId.isValidNamespace("__12"));
+        assertTrue(ShapeId.isValidNamespace("__1_"));
+        assertFalse(ShapeId.isValidNamespace("__1#"));
+
+        // Identifier: ALPHA *`IdentifierChars`
+        assertTrue(ShapeId.isValidNamespace("ab"));
+        assertTrue(ShapeId.isValidNamespace("a1"));
+        assertTrue(ShapeId.isValidNamespace("a_"));
+        assertFalse(ShapeId.isValidNamespace("a#"));
+    }
+
     @ParameterizedTest
     @MethodSource("shapeIdData")
     public void ShapeIdValidationsTest(String shapeId, boolean isInvalid) {