diff --git a/packages/util-endpoints/src/env-endpoint.integ.spec.ts b/packages/util-endpoints/src/env-endpoint.integ.spec.ts
new file mode 100644
index 000000000000..ba196ac4f076
--- /dev/null
+++ b/packages/util-endpoints/src/env-endpoint.integ.spec.ts
@@ -0,0 +1,90 @@
+import { DynamoDB } from "@aws-sdk/client-dynamodb";
+import { S3 } from "@aws-sdk/client-s3";
+import { afterAll, beforeEach, describe, test as it } from "vitest";
+
+import { requireRequestsFrom } from "../../../private/aws-util-test/src";
+
+/**
+ * This is an AWS-specific integration test for a piece of functionality that is contained
+ * within `@smithy/middleware-endpoint` (another repository).
+ *
+ * It augments unit tests in that package.
+ */
+describe("environment variable custom endpoint", () => {
+  beforeEach(async () => {
+    delete process.env.AWS_ENDPOINT_URL;
+    delete process.env.AWS_ENDPOINT_URL_DYNAMODB;
+  });
+
+  afterAll(async () => {
+    delete process.env.AWS_ENDPOINT_URL;
+    delete process.env.AWS_ENDPOINT_URL_DYNAMODB;
+  });
+
+  it("should override a client endpoint if none is set on the client", async () => {
+    process.env.AWS_ENDPOINT_URL = "https://localhost/1";
+    const s3 = new S3({
+      region: "us-west-2",
+    });
+    const ddb = new DynamoDB({
+      region: "us-west-2",
+    });
+
+    requireRequestsFrom(s3).toMatch({
+      hostname: /localhost/,
+      path: /\/1/,
+    });
+    requireRequestsFrom(ddb).toMatch({
+      hostname: /localhost/,
+      path: /\/1/,
+    });
+
+    await s3.listBuckets();
+    await ddb.listTables();
+  });
+
+  it("should defer to the client's manually set endpoint", async () => {
+    process.env.AWS_ENDPOINT_URL = "https://localhost/1";
+    const s3 = new S3({
+      region: "us-west-2",
+      endpoint: "https://localhost/2",
+    });
+    const ddb = new DynamoDB({
+      region: "us-west-2",
+      endpoint: "https://localhost/2",
+    });
+
+    requireRequestsFrom(s3).toMatch({
+      hostname: /localhost/,
+      path: /\/2/,
+    });
+    requireRequestsFrom(ddb).toMatch({
+      hostname: /localhost/,
+      path: /\/2/,
+    });
+
+    await s3.listBuckets();
+    await ddb.listTables();
+  });
+
+  it("should allow a specific service id to be chosen in the env variable name", async () => {
+    process.env.AWS_ENDPOINT_URL_DYNAMODB = "https://localhost/1";
+    const s3 = new S3({
+      region: "us-west-2",
+    });
+    const ddb = new DynamoDB({
+      region: "us-west-2",
+    });
+
+    requireRequestsFrom(s3).toMatch({
+      hostname: /s3\.us-west-2\.amazonaws\.com/,
+    });
+    requireRequestsFrom(ddb).toMatch({
+      hostname: /localhost/,
+      path: /\/1/,
+    });
+
+    await s3.listBuckets();
+    await ddb.listTables();
+  });
+});