diff --git a/assert/unreachable.ts b/assert/unreachable.ts
index 6f57feed199a..e58eb81f546a 100644
--- a/assert/unreachable.ts
+++ b/assert/unreachable.ts
@@ -12,9 +12,10 @@ import { AssertionError } from "./assertion_error.ts";
  * unreachable(); // Throws
  * ```
  *
- * @param reason The reason why the code should be unreachable.
+ * @param msg Optional message to include in the error.
  * @returns Never returns, always throws.
  */
-export function unreachable(reason?: string): never {
-  throw new AssertionError(reason ?? "unreachable");
+export function unreachable(msg?: string): never {
+  const msgSuffix = msg ? `: ${msg}` : ".";
+  throw new AssertionError(`Unreachable${msgSuffix}`);
 }
diff --git a/assert/unreachable_test.ts b/assert/unreachable_test.ts
index 3097c79ebb95..a35a5a26a0f8 100644
--- a/assert/unreachable_test.ts
+++ b/assert/unreachable_test.ts
@@ -2,10 +2,10 @@
 import { AssertionError, assertThrows, unreachable } from "./mod.ts";
 
 Deno.test("unreachable()", () => {
-  assertThrows(() => unreachable(), AssertionError, "unreachable");
+  assertThrows(() => unreachable(), AssertionError, "Unreachable.");
   assertThrows(
     () => unreachable("custom message"),
     AssertionError,
-    "custom message",
+    "Unreachable: custom message",
   );
 });