diff --git a/rules/aip0123/resource_pattern.go b/rules/aip0123/resource_pattern.go
index f0c7bc5d0..8a378160a 100644
--- a/rules/aip0123/resource_pattern.go
+++ b/rules/aip0123/resource_pattern.go
@@ -46,9 +46,11 @@ func lintResourcePattern(resource *annotations.ResourceDescriptor, desc desc.Des
 	}
 
 	// Ensure that the constant segments of the pattern uses camel case,
-	// not snake case.
+	// not snake case, and there are no spaces.
 	for _, pattern := range resource.GetPattern() {
-		if strings.Contains(getPlainPattern(pattern), "_") {
+		plainPattern := getPlainPattern(pattern)
+
+		if strings.Contains(plainPattern, "_") {
 			return []lint.Problem{{
 				Message: fmt.Sprintf(
 					"Resource patterns should use camel case (apart from the variable names), such as %q.",
@@ -58,6 +60,13 @@ func lintResourcePattern(resource *annotations.ResourceDescriptor, desc desc.Des
 				Location:   loc,
 			}}
 		}
+		if strings.Contains(plainPattern, " ") {
+			return []lint.Problem{{
+				Message:    "Resource patterns should not have spaces",
+				Descriptor: desc,
+				Location:   loc,
+			}}
+		}
 	}
 	return nil
 }
diff --git a/rules/aip0123/resource_pattern_test.go b/rules/aip0123/resource_pattern_test.go
index 65956557d..276de85ee 100644
--- a/rules/aip0123/resource_pattern_test.go
+++ b/rules/aip0123/resource_pattern_test.go
@@ -32,6 +32,9 @@ func TestResourcePattern(t *testing.T) {
 		{"SnakeCase", `pattern: "book_publishers/{book_publisher}/books/{book}"`, testutils.Problems{{
 			Message: "bookPublishers/{book_publisher}/books/{book}",
 		}}},
+		{"HasSpaces", `pattern: "publishers/{publisher}/ books /{book}"`, testutils.Problems{{
+			Message: "Resource patterns should not have spaces",
+		}}},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			f := testutils.ParseProto3Tmpl(t, `