diff --git a/aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/RegionDecorator.kt b/aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/RegionDecorator.kt
index 5dacc952bd..25c72ee3ef 100644
--- a/aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/RegionDecorator.kt
+++ b/aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/RegionDecorator.kt
@@ -5,6 +5,8 @@
 
 package software.amazon.smithy.rustsdk
 
+import software.amazon.smithy.aws.traits.auth.SigV4Trait
+import software.amazon.smithy.model.knowledge.ServiceIndex
 import software.amazon.smithy.model.node.Node
 import software.amazon.smithy.model.shapes.OperationShape
 import software.amazon.smithy.rulesengine.language.syntax.parameters.Builtins
@@ -82,7 +84,11 @@ class RegionDecorator : ClientCodegenDecorator {
     override val name: String = "Region"
     override val order: Byte = 0
 
-    private fun usesRegion(codegenContext: ClientCodegenContext) = codegenContext.getBuiltIn(Builtins.REGION) != null
+    // Services that have an endpoint ruleset that references the SDK::Region built in, or
+    // that use SigV4, both need a configurable region.
+    private fun usesRegion(codegenContext: ClientCodegenContext) =
+        codegenContext.getBuiltIn(Builtins.REGION) != null || ServiceIndex.of(codegenContext.model)
+            .getEffectiveAuthSchemes(codegenContext.serviceShape).containsKey(SigV4Trait.ID)
 
     override fun configCustomizations(
         codegenContext: ClientCodegenContext,
diff --git a/aws/sdk-codegen/src/test/kotlin/software/amazon/smithy/rustsdk/RegionDecoratorTest.kt b/aws/sdk-codegen/src/test/kotlin/software/amazon/smithy/rustsdk/RegionDecoratorTest.kt
new file mode 100644
index 0000000000..0900142258
--- /dev/null
+++ b/aws/sdk-codegen/src/test/kotlin/software/amazon/smithy/rustsdk/RegionDecoratorTest.kt
@@ -0,0 +1,105 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package software.amazon.smithy.rustsdk
+
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
+import kotlin.io.path.readText
+
+class RegionDecoratorTest {
+    private val modelWithoutRegionParamOrSigV4AuthScheme = """
+        namespace test
+
+        use aws.api#service
+        use aws.protocols#awsJson1_0
+        use smithy.rules#endpointRuleSet
+
+        @awsJson1_0
+        @endpointRuleSet({
+            "version": "1.0",
+            "rules": [{ "type": "endpoint", "conditions": [], "endpoint": { "url": "https://example.com" } }],
+            "parameters": {}
+        })
+        @service(sdkId: "dontcare")
+        service TestService { version: "2023-01-01", operations: [SomeOperation] }
+        structure SomeOutput { something: String }
+        operation SomeOperation { output: SomeOutput }
+    """.asSmithyModel()
+
+    private val modelWithRegionParam = """
+        namespace test
+
+        use aws.api#service
+        use aws.protocols#awsJson1_0
+        use smithy.rules#endpointRuleSet
+
+        @awsJson1_0
+        @endpointRuleSet({
+            "version": "1.0",
+            "rules": [{ "type": "endpoint", "conditions": [], "endpoint": { "url": "https://example.com" } }],
+            "parameters": {
+                "Region": { "required": false, "type": "String", "builtIn": "AWS::Region" },
+            }
+        })
+        @service(sdkId: "dontcare")
+        service TestService { version: "2023-01-01", operations: [SomeOperation] }
+        structure SomeOutput { something: String }
+        operation SomeOperation { output: SomeOutput }
+    """.asSmithyModel()
+
+    private val modelWithSigV4AuthScheme = """
+        namespace test
+
+        use aws.auth#sigv4
+        use aws.api#service
+        use aws.protocols#awsJson1_0
+        use smithy.rules#endpointRuleSet
+
+        @auth([sigv4])
+        @sigv4(name: "dontcare")
+        @awsJson1_0
+        @endpointRuleSet({
+            "version": "1.0",
+            "rules": [{ "type": "endpoint", "conditions": [], "endpoint": { "url": "https://example.com" } }],
+            "parameters": {}
+        })
+        @service(sdkId: "dontcare")
+        service TestService { version: "2023-01-01", operations: [SomeOperation] }
+        structure SomeOutput { something: String }
+        operation SomeOperation { output: SomeOutput }
+    """.asSmithyModel()
+
+    @Test
+    fun `models without region built-in params or SigV4 should not have configurable regions`() {
+        val path = awsSdkIntegrationTest(
+            modelWithoutRegionParamOrSigV4AuthScheme,
+            generateOrchestrator = true,
+        ) { _, _ -> /* it should generate and compile successfully */ }
+        val configContents = path.resolve("src/config.rs").readText()
+        assertFalse(configContents.contains("fn set_region("))
+    }
+
+    @Test
+    fun `models with region built-in params should have configurable regions`() {
+        val path = awsSdkIntegrationTest(
+            modelWithRegionParam,
+            generateOrchestrator = true,
+        ) { _, _ -> /* it should generate and compile successfully */ }
+        val configContents = path.resolve("src/config.rs").readText()
+        assertTrue(configContents.contains("fn set_region("))
+    }
+
+    @Test
+    fun `models with SigV4 should have configurable regions`() {
+        val path = awsSdkIntegrationTest(
+            modelWithSigV4AuthScheme,
+            generateOrchestrator = true,
+        ) { _, _ -> /* it should generate and compile successfully */ }
+        val configContents = path.resolve("src/config.rs").readText()
+        assertTrue(configContents.contains("fn set_region("))
+    }
+}