diff --git a/doc/config-schema.md b/doc/config-schema.md
index 4d507c0ac26..1a5194b72f4 100644
--- a/doc/config-schema.md
+++ b/doc/config-schema.md
@@ -299,6 +299,7 @@ This document describes the schema for the librarian.yaml.
 
 | Field | Type | Description |
 | :--- | :--- | :--- |
+| `custom_group_ids` | map[string]string | Maps API path prefixes (e.g., "google/shopping") to their corresponding Maven Group IDs (e.g., "com.google.shopping"). Use this to override the default "com.google.cloud" Group ID for specific API paths (e.g., maps, ads, shopping). |
 | `libraries_bom_version` | string | Is the version of the libraries-bom to use for Java. |
 
 ## JavaFileCopy Configuration
diff --git a/internal/config/language.go b/internal/config/language.go
index bd72cb1e373..91fc3c3d92a 100644
--- a/internal/config/language.go
+++ b/internal/config/language.go
@@ -458,6 +458,11 @@ type DartPackage struct {
 
 // JavaDefault contains Java-specific default configuration.
 type JavaDefault struct {
+	// CustomGroupIDs maps API path prefixes (e.g., "google/shopping") to their
+	// corresponding Maven Group IDs (e.g., "com.google.shopping").
+	// Use this to override the default "com.google.cloud" Group ID for specific API
+	// paths (e.g., maps, ads, shopping).
+	CustomGroupIDs map[string]string `yaml:"custom_group_ids,omitempty"`
 	// LibrariesBOMVersion is the version of the libraries-bom to use for Java.
 	LibrariesBOMVersion string `yaml:"libraries_bom_version,omitempty"`
 }
diff --git a/internal/librarian/library.go b/internal/librarian/library.go
index 140ab30e849..1f9e834746f 100644
--- a/internal/librarian/library.go
+++ b/internal/librarian/library.go
@@ -44,22 +44,22 @@ func fillDefaults(lib *config.Library, d *config.Default) *config.Library {
 	if lib.Output == "" {
 		lib.Output = d.Output
 	}
-	if d.Go != nil {
+	switch {
+	case d.Go != nil:
 		return fillGo(lib, d)
-	}
-	if d.Rust != nil {
+	case d.Java != nil:
+		return fillJava(lib, d)
+	case d.Rust != nil:
 		return fillRust(lib, d)
-	}
-	if d.Dart != nil {
+	case d.Dart != nil:
 		return fillDart(lib, d)
-	}
-	if d.Python != nil {
+	case d.Python != nil:
 		return fillPython(lib, d)
-	}
-	if d.Swift != nil {
+	case d.Swift != nil:
 		return fillSwift(lib, d)
+	default:
+		return lib
 	}
-	return lib
 }
 
 // fillGo populates empty Go-specific fields in lib from the provided default.
@@ -97,6 +97,32 @@ func union(a, b []string) []string {
 	return res
 }
 
+// fillJava populates empty Java-specific fields in lib from the provided default.
+func fillJava(lib *config.Library, d *config.Default) *config.Library {
+	if lib.Java == nil {
+		lib.Java = &config.JavaModule{}
+	}
+	fillGroupIDIfEmpty(lib, d)
+	return lib
+}
+
+// fillGroupIDIfEmpty sets the Java group ID on lib if one is not already configured.
+// It matches the library's API paths against the custom group ID prefixes in default
+// and assigns the first matching group ID.
+func fillGroupIDIfEmpty(lib *config.Library, d *config.Default) {
+	if lib.Java.GroupID != "" || d.Java.CustomGroupIDs == nil {
+		return
+	}
+	for _, api := range lib.APIs {
+		for apiPrefix, groupID := range d.Java.CustomGroupIDs {
+			if api.Path == apiPrefix || strings.HasPrefix(api.Path, apiPrefix+"/") {
+				lib.Java.GroupID = groupID
+				return
+			}
+		}
+	}
+}
+
 // fillRust populates empty Rust-specific fields in lib from the provided default.
 func fillRust(lib *config.Library, d *config.Default) *config.Library {
 	if lib.Rust == nil {
diff --git a/internal/librarian/library_test.go b/internal/librarian/library_test.go
index 464e97654d4..237d6e3b997 100644
--- a/internal/librarian/library_test.go
+++ b/internal/librarian/library_test.go
@@ -233,6 +233,217 @@ func TestFillDefaults(t *testing.T) {
 	}
 }
 
+func TestFillDefaults_Java(t *testing.T) {
+	defaults := &config.Default{
+		Java: &config.JavaDefault{
+			CustomGroupIDs: map[string]string{
+				"google/shopping":  "com.google.shopping",
+				"google/exact/v1":  "com.google.exact",
+				"google/maps":      "com.google.maps",
+				"google/ads":       "com.google.api-ads",
+				"google/analytics": "com.google.analytics",
+			},
+		},
+	}
+	for _, test := range []struct {
+		name     string
+		lib      *config.Library
+		defaults *config.Default
+		want     *config.Library
+	}{
+		{
+			name: "shopping library",
+			lib: &config.Library{
+				Name: "shopping-merchant-issue-resolution",
+				APIs: []*config.API{
+					{Path: "google/shopping/merchant/issueresolution/v1"},
+					{Path: "google/shopping/merchant/issueresolution/v1beta"},
+				},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "shopping-merchant-issue-resolution",
+				APIs: []*config.API{
+					{Path: "google/shopping/merchant/issueresolution/v1"},
+					{Path: "google/shopping/merchant/issueresolution/v1beta"},
+				},
+				Java: &config.JavaModule{
+					GroupID: "com.google.shopping",
+				},
+			},
+		},
+		{
+			name: "do not override custom artifact id",
+			lib: &config.Library{
+				Name: "custom-shopping",
+				APIs: []*config.API{
+					{Path: "google/shopping/merchant/issueresolution/v1"},
+					{Path: "google/shopping/merchant/issueresolution/v1beta"},
+				},
+				Java: &config.JavaModule{
+					ArtifactID: "custom-shopping-id",
+				},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "custom-shopping",
+				APIs: []*config.API{
+					{Path: "google/shopping/merchant/issueresolution/v1"},
+					{Path: "google/shopping/merchant/issueresolution/v1beta"},
+				},
+				Java: &config.JavaModule{
+					ArtifactID: "custom-shopping-id",
+					GroupID:    "com.google.shopping",
+				},
+			},
+		},
+		{
+			name: "maps library",
+			lib: &config.Library{
+				Name: "maps-routeoptimization",
+				APIs: []*config.API{{Path: "google/maps/routeoptimization/v1"}},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "maps-routeoptimization",
+				APIs: []*config.API{{Path: "google/maps/routeoptimization/v1"}},
+				Java: &config.JavaModule{
+					GroupID: "com.google.maps",
+				},
+			},
+		},
+		{
+			name: "ads library",
+			lib: &config.Library{
+				Name: "admanager",
+				APIs: []*config.API{{Path: "google/ads/admanager/v1"}},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "admanager",
+				APIs: []*config.API{{Path: "google/ads/admanager/v1"}},
+				Java: &config.JavaModule{
+					GroupID: "com.google.api-ads",
+				},
+			},
+		},
+		{
+			name: "analytics library",
+			lib: &config.Library{
+				Name: "analytics-admin",
+				APIs: []*config.API{
+					{Path: "google/analytics/admin/v1beta"},
+					{Path: "google/analytics/admin/v1alpha"},
+				},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "analytics-admin",
+				APIs: []*config.API{
+					{Path: "google/analytics/admin/v1beta"},
+					{Path: "google/analytics/admin/v1alpha"},
+				},
+				Java: &config.JavaModule{
+					GroupID: "com.google.analytics",
+				},
+			},
+		},
+		{
+			name: "do not fill if group id already set",
+			lib: &config.Library{
+				Name: "common-protos",
+				APIs: []*config.API{
+					{Path: "google/shopping/type"},
+				},
+				Java: &config.JavaModule{
+					GroupID: "com.google.api.grpc",
+				},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "common-protos",
+				APIs: []*config.API{
+					{Path: "google/shopping/type"},
+				},
+				Java: &config.JavaModule{
+					GroupID: "com.google.api.grpc",
+				},
+			},
+		},
+		{
+			name: "prefix match respects path segment boundaries",
+			lib: &config.Library{
+				Name: "shopping-foo",
+				APIs: []*config.API{
+					{Path: "google/shopping-foo/v1"},
+				},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "shopping-foo",
+				APIs: []*config.API{
+					{Path: "google/shopping-foo/v1"},
+				},
+				Java: &config.JavaModule{},
+			},
+		},
+		{
+			name: "no matching api prefix leaves group id empty",
+			lib: &config.Library{
+				Name: "unknown",
+				APIs: []*config.API{{Path: "google/unknown/v1"}},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "unknown",
+				APIs: []*config.API{{Path: "google/unknown/v1"}},
+				Java: &config.JavaModule{},
+			},
+		},
+		{
+			name: "library does not change with nil map",
+			lib: &config.Library{
+				Name: "lib",
+				APIs: []*config.API{{Path: "google/example/v1"}},
+			},
+			defaults: &config.Default{
+				Java: &config.JavaDefault{},
+			},
+			want: &config.Library{
+				Name: "lib",
+				APIs: []*config.API{{Path: "google/example/v1"}},
+				Java: &config.JavaModule{},
+			},
+		},
+		{
+			name: "api path exact match",
+			lib: &config.Library{
+				Name: "exact",
+				APIs: []*config.API{
+					{Path: "google/exact/v1"},
+				},
+			},
+			defaults: defaults,
+			want: &config.Library{
+				Name: "exact",
+				APIs: []*config.API{
+					{Path: "google/exact/v1"},
+				},
+				Java: &config.JavaModule{
+					GroupID: "com.google.exact",
+				},
+			},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			got := fillJava(test.lib, test.defaults)
+			if diff := cmp.Diff(test.want, got); diff != "" {
+				t.Errorf("mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestFillDefaults_Rust(t *testing.T) {
 	defaults := &config.Default{
 		Rust: &config.RustDefault{