diff --git a/packages/sdk/src/core/client.ts b/packages/sdk/src/core/client.ts
index 7e7b961..5c86946 100644
--- a/packages/sdk/src/core/client.ts
+++ b/packages/sdk/src/core/client.ts
@@ -22,8 +22,21 @@ const defaultClock = {
   },
 };
 
+function assertServerEnvironment(): void {
+  const isBrowser =
+    typeof window !== "undefined" ||
+    typeof document !== "undefined";
+
+  if (isBrowser) {
+    throw new Error(
+      "Domain SDK is server-side only. Initialize it in a server route, server action, or backend service.",
+    );
+  }
+}
+
 /** Create a stateless custom-domain client backed by one provider adapter. */
 export function createDomainClient(options: DomainClientOptions): DomainClient {
+  assertServerEnvironment();
   if (!options?.provider)
     throw new DomainSdkError("INVALID_CONFIGURATION", "A domain provider is required.");
   const { provider } = options;
diff --git a/packages/sdk/test/core/client.test.ts b/packages/sdk/test/core/client.test.ts
index 31f5215..5a7df1f 100644
--- a/packages/sdk/test/core/client.test.ts
+++ b/packages/sdk/test/core/client.test.ts
@@ -3,6 +3,20 @@ import { createDomainClient, DomainSdkError } from "../../src";
 import { createMockDnsRecord, memoryProvider } from "../../src/testing";
 
 describe("domain client and memory provider", () => {
+  test("rejects initialization in a browser environment", () => {
+    const windowDescriptor = Object.getOwnPropertyDescriptor(globalThis, "window");
+    Object.defineProperty(globalThis, "window", { value: {}, configurable: true });
+
+    try {
+      expect(() => createDomainClient({ provider: memoryProvider() })).toThrow(
+        "Domain SDK is server-side only",
+      );
+    } finally {
+      if (windowDescriptor) Object.defineProperty(globalThis, "window", windowDescriptor);
+      else Reflect.deleteProperty(globalThis, "window");
+    }
+  });
+
   test("supports the lifecycle and duplicate add", async () => {
     const memory = memoryProvider({ records: [createMockDnsRecord(), createMockDnsRecord()] });
     const domains = createDomainClient({ provider: memory });