diff --git a/.changes/5c7e4dc6-7bf9-421b-a6d9-01007eacf6be.json b/.changes/5c7e4dc6-7bf9-421b-a6d9-01007eacf6be.json
new file mode 100644
index 00000000000..c2999bf799b
--- /dev/null
+++ b/.changes/5c7e4dc6-7bf9-421b-a6d9-01007eacf6be.json
@@ -0,0 +1,8 @@
+{
+    "id": "5c7e4dc6-7bf9-421b-a6d9-01007eacf6be",
+    "type": "misc",
+    "description": "Enable [Explicit API mode](https://github.com/Kotlin/KEEP/blob/master/proposals/explicit-api-mode.md)",
+    "issues": [
+        "awslabs/smithy-kotlin#216"
+    ]
+}
\ No newline at end of file
diff --git a/aws-runtime/build.gradle.kts b/aws-runtime/build.gradle.kts
index bfd90cad864..5cb7a55b3f5 100644
--- a/aws-runtime/build.gradle.kts
+++ b/aws-runtime/build.gradle.kts
@@ -62,8 +62,7 @@ subprojects {
     }
 
     kotlin {
-        // TODO - refactor this to error `explicitApi()`
-        explicitApiWarning()
+        explicitApi()
 
         sourceSets {
             all {
diff --git a/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/PresignerGenerator.kt b/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/PresignerGenerator.kt
index 344e8c5be13..a7b8d737863 100644
--- a/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/PresignerGenerator.kt
+++ b/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/PresignerGenerator.kt
@@ -317,8 +317,10 @@ class PresignerGenerator : KotlinIntegration {
         writer
             .addImport(KotlinTypes.Time.Duration)
             .withBlock(
-                "suspend fun $requestTypeName.presign(config: $serviceClientTypeName.Config, duration: #T): HttpRequest {",
+                "public suspend fun #L.presign(config: #L.Config, duration: #T): HttpRequest {",
                 "}\n",
+                requestTypeName,
+                serviceClientTypeName,
                 KotlinTypes.Time.Duration
             ) {
                 withBlock("val presignConfig = $presignConfigTypeName {", "}") {
@@ -341,8 +343,9 @@ class PresignerGenerator : KotlinIntegration {
         writer
             .addImport(KotlinTypes.Time.Duration)
             .withBlock(
-                "suspend fun $requestTypeName.presign(presignConfig: ServicePresignConfig, duration: #T): HttpRequest {",
+                "public suspend fun #L.presign(presignConfig: ServicePresignConfig, duration: #T): HttpRequest {",
                 "}\n",
+                requestTypeName,
                 KotlinTypes.Time.Duration
             ) {
                 write("return createPresignedRequest(presignConfig, $requestConfigFnName(this, duration))")
diff --git a/codegen/smithy-aws-kotlin-codegen/src/test/kotlin/aws/sdk/kotlin/codegen/PresignerGeneratorTest.kt b/codegen/smithy-aws-kotlin-codegen/src/test/kotlin/aws/sdk/kotlin/codegen/PresignerGeneratorTest.kt
index 4c4942674c6..71014722a3d 100644
--- a/codegen/smithy-aws-kotlin-codegen/src/test/kotlin/aws/sdk/kotlin/codegen/PresignerGeneratorTest.kt
+++ b/codegen/smithy-aws-kotlin-codegen/src/test/kotlin/aws/sdk/kotlin/codegen/PresignerGeneratorTest.kt
@@ -116,7 +116,7 @@ class PresignerGeneratorTest {
              * @param duration the amount of time from signing for which the request is valid, with seconds granularity.
              * @return The [HttpRequest] that can be invoked within the specified time window.
              */
-            suspend fun GetFooRequest.presign(presignConfig: ServicePresignConfig, duration: Duration): HttpRequest {
+            public suspend fun GetFooRequest.presign(presignConfig: ServicePresignConfig, duration: Duration): HttpRequest {
                 return createPresignedRequest(presignConfig, getFooPresignConfig(this, duration))
             }
             
@@ -126,7 +126,7 @@ class PresignerGeneratorTest {
              * @param duration the amount of time from signing for which the request is valid, with seconds granularity.
              * @return The [HttpRequest] that can be invoked within the specified time window.
              */
-            suspend fun GetFooRequest.presign(config: TestClient.Config, duration: Duration): HttpRequest {
+            public suspend fun GetFooRequest.presign(config: TestClient.Config, duration: Duration): HttpRequest {
                 val presignConfig = TestPresignConfig {
                     credentialsProvider = config.credentialsProvider
                     endpointProvider = config.endpointResolver.asSigningEndpointProvider()
@@ -155,7 +155,7 @@ class PresignerGeneratorTest {
              * @param duration the amount of time from signing for which the request is valid, with seconds granularity.
              * @return The [HttpRequest] that can be invoked within the specified time window.
              */
-            suspend fun PostFooRequest.presign(presignConfig: ServicePresignConfig, duration: Duration): HttpRequest {
+            public suspend fun PostFooRequest.presign(presignConfig: ServicePresignConfig, duration: Duration): HttpRequest {
                 return createPresignedRequest(presignConfig, postFooPresignConfig(this, duration))
             }
             
@@ -165,7 +165,7 @@ class PresignerGeneratorTest {
              * @param duration the amount of time from signing for which the request is valid, with seconds granularity.
              * @return The [HttpRequest] that can be invoked within the specified time window.
              */
-            suspend fun PostFooRequest.presign(config: TestClient.Config, duration: Duration): HttpRequest {
+            public suspend fun PostFooRequest.presign(config: TestClient.Config, duration: Duration): HttpRequest {
                 val presignConfig = TestPresignConfig {
                     credentialsProvider = config.credentialsProvider
                     endpointProvider = config.endpointResolver.asSigningEndpointProvider()
@@ -194,7 +194,7 @@ class PresignerGeneratorTest {
              * @param duration the amount of time from signing for which the request is valid, with seconds granularity.
              * @return The [HttpRequest] that can be invoked within the specified time window.
              */
-            suspend fun PutFooRequest.presign(presignConfig: ServicePresignConfig, duration: Duration): HttpRequest {
+            public suspend fun PutFooRequest.presign(presignConfig: ServicePresignConfig, duration: Duration): HttpRequest {
                 return createPresignedRequest(presignConfig, putFooPresignConfig(this, duration))
             }
             
@@ -204,7 +204,7 @@ class PresignerGeneratorTest {
              * @param duration the amount of time from signing for which the request is valid, with seconds granularity.
              * @return The [HttpRequest] that can be invoked within the specified time window.
              */
-            suspend fun PutFooRequest.presign(config: TestClient.Config, duration: Duration): HttpRequest {
+            public suspend fun PutFooRequest.presign(config: TestClient.Config, duration: Duration): HttpRequest {
                 val presignConfig = TestPresignConfig {
                     credentialsProvider = config.credentialsProvider
                     endpointProvider = config.endpointResolver.asSigningEndpointProvider()
diff --git a/services/build.gradle.kts b/services/build.gradle.kts
index 85237d4e155..c39fd0ecc0e 100644
--- a/services/build.gradle.kts
+++ b/services/build.gradle.kts
@@ -44,6 +44,8 @@ subprojects {
     }
 
     kotlin {
+        explicitApi()
+
         sourceSets {
             all {
                 val srcDir = if (name.endsWith("Main")) "src" else "test"