diff --git a/.size-limit.json b/.size-limit.json
index 47600c3..18ab8c5 100644
--- a/.size-limit.json
+++ b/.size-limit.json
@@ -41,6 +41,12 @@
     "limit": "195 B",
     "brotli": true
   },
+  {
+    "name": "@peerigon/typescript-toolkit/assert-never",
+    "path": "dist/assert-never/assert-never.js",
+    "limit": "200 B",
+    "brotli": true
+  },
   {
     "name": "@peerigon/typescript-toolkit/dedupe",
     "path": "dist/dedupe/dedupe.js",
diff --git a/README.md b/README.md
index d2fd77b..63c0dee 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,7 @@ import { assert } from "@peerigon/typescript-toolkit/assert";
 | [`api/rate-limit`](./src/api/rate-limit/README.md)                                 | Rate-limited `fetch` for `defineApi` (pacing + Retry-After)                        | [→](./src/api/rate-limit/README.md)                 |
 | [`api/result`](./src/api/result/README.md)                                         | `defineApiResult` — like `defineApi`, but returns `Result.Sync`                    | [→](./src/api/result/README.md)                     |
 | [`assert`](./src/assert/README.md)                                                 | Assert a value is not `null` or `undefined`, with TypeScript narrowing             | [→](./src/assert/README.md)                         |
+| [`assert-never`](./src/assert-never/README.md)                                     | Assert a code path is unreachable, for exhaustive `switch`/`case` statements       | [→](./src/assert-never/README.md)                   |
 | [`concurrency/once`](./src/concurrency/once/README.md)                             | Run an async function at most once (single-flight + cache)                         | [→](./src/concurrency/once/README.md)               |
 | [`concurrency/once/result`](./src/concurrency/once/result/README.md)               | `once` with a synchronous `Result` snapshot                                        | [→](./src/concurrency/once/result/README.md)        |
 | [`concurrency/exactlyOnce`](./src/concurrency/exactlyOnce/README.md)               | Invoke an async function exactly once; second call throws                          | [→](./src/concurrency/exactlyOnce/README.md)        |
diff --git a/jsr.json b/jsr.json
index 5fd3d75..760e7cd 100644
--- a/jsr.json
+++ b/jsr.json
@@ -7,6 +7,7 @@
     "./api/rate-limit": "./src/api/rate-limit/rate-limit.ts",
     "./api/result": "./src/api/result/result.ts",
     "./assert": "./src/assert/assert.ts",
+    "./assert-never": "./src/assert-never/assert-never.ts",
     "./concurrency/exactlyOnce": "./src/concurrency/exactlyOnce/exactlyOnce.ts",
     "./concurrency/exactlyOnce/result": "./src/concurrency/exactlyOnce/result/result.ts",
     "./concurrency/mutex": "./src/concurrency/mutex/mutex.ts",
diff --git a/package.json b/package.json
index 8b888ce..505ea83 100644
--- a/package.json
+++ b/package.json
@@ -20,6 +20,7 @@
     "./api/rate-limit": "./dist/api/rate-limit/rate-limit.js",
     "./api/result": "./dist/api/result/result.js",
     "./assert": "./dist/assert/assert.js",
+    "./assert-never": "./dist/assert-never/assert-never.js",
     "./concurrency/exactlyOnce": "./dist/concurrency/exactlyOnce/exactlyOnce.js",
     "./concurrency/exactlyOnce/result": "./dist/concurrency/exactlyOnce/result/result.js",
     "./concurrency/mutex": "./dist/concurrency/mutex/mutex.js",
diff --git a/src/assert-never/README.md b/src/assert-never/README.md
new file mode 100644
index 0000000..16cd8d7
--- /dev/null
+++ b/src/assert-never/README.md
@@ -0,0 +1,59 @@
+## `assertNever`
+
+- 📦 Below 200 Bytes minified + compressed (brotli)
+- ✅ Zero dependencies
+
+Assert that a code path is unreachable, typically the `default` case of a `switch`/`case` statement over a union type.
+
+`assertNever` only accepts a value whose type has already been narrowed to `never`. If you handle every case of a union, TypeScript narrows the value in the `default` branch to `never` and the call compiles. If the union later gains a new member, TypeScript reports a compile-time error at the `assertNever` call site instead of silently falling through. At runtime, `assertNever` throws, guarding against values that slip through despite the type system (e.g. from `JSON.parse` or an external API).
+
+### Basic usage
+
+```ts
+import { assertNever } from "@peerigon/typescript-toolkit/assert-never";
+
+type Direction = "down" | "up";
+
+const describeDirection = (direction: Direction): string => {
+  switch (direction) {
+    case "up":
+      return "going up";
+    case "down":
+      return "going down";
+    default:
+      // TypeScript error here if a case for "Direction" is missing
+      return assertNever(direction);
+  }
+};
+```
+
+### With custom error message
+
+```ts
+default:
+  return assertNever(direction, "Unhandled direction");
+
+// Custom error messages just for the development build. Production builds will remove the message. In that case, a generic default error message is used.
+default:
+  return assertNever(
+    direction,
+    import.meta.env.DEV && `Unhandled direction: ${direction}`,
+  );
+```
+
+### API Reference
+
+#### `assertNever(value, errorMessage?)`
+
+Asserts that `value` is of type `never` and throws at runtime.
+
+```ts
+assertNever(value: never, errorMessage?: ErrorMessage): never
+```
+
+| Parameter      | Type                      | Description                                                                             |
+| -------------- | ------------------------- | --------------------------------------------------------------------------------------- |
+| `value`        | `never`                   | Value that should be unreachable, i.e. all union members have already been handled      |
+| `errorMessage` | `ErrorMessage` (optional) | Custom message: `string`, `false`, or a lazy function. Default: `"Unexpected value: …"` |
+
+**Throws:** `TypeError` unconditionally, since reaching this function is always a bug or an unexpected runtime value.
diff --git a/src/assert-never/assert-never.test.ts b/src/assert-never/assert-never.test.ts
new file mode 100644
index 0000000..a3d82f5
--- /dev/null
+++ b/src/assert-never/assert-never.test.ts
@@ -0,0 +1,60 @@
+import { describe, expect, expectTypeOf, it } from "vitest";
+import { assertNever } from "./assert-never.js";
+
+describe("assertNever()", () => {
+  it("throws for any given value", () => {
+    expect(() =>
+      assertNever("unexpected" as never),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TypeError: Unexpected value: "unexpected"]`,
+    );
+  });
+
+  it("throws for objects", () => {
+    expect(() =>
+      assertNever({ type: "unknown" } as never),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TypeError: Unexpected value: {"type":"unknown"}]`,
+    );
+  });
+
+  it("uses custom message when provided", () => {
+    expect(() =>
+      assertNever("unexpected" as never, "Custom assertion error"),
+    ).toThrow("Custom assertion error");
+  });
+
+  it("calls the function message when provided", () => {
+    const messageFn = () => "Dynamic error message";
+    expect(() => assertNever("unexpected" as never, messageFn)).toThrow(
+      "Dynamic error message",
+    );
+  });
+
+  describe("exhaustiveness checking", () => {
+    type Direction = "down" | "up";
+
+    const describeDirection = (direction: Direction): string => {
+      switch (direction) {
+        case "up": {
+          return "going up";
+        }
+        case "down": {
+          return "going down";
+        }
+        default: {
+          return assertNever(direction);
+        }
+      }
+    };
+
+    it("handles every case without reaching the default branch", () => {
+      expect(describeDirection("up")).toBe("going up");
+      expect(describeDirection("down")).toBe("going down");
+    });
+
+    it("only accepts a value of type never", () => {
+      expectTypeOf(assertNever).parameter(0).toEqualTypeOf<never>();
+    });
+  });
+});
diff --git a/src/assert-never/assert-never.ts b/src/assert-never/assert-never.ts
new file mode 100644
index 0000000..7b0d114
--- /dev/null
+++ b/src/assert-never/assert-never.ts
@@ -0,0 +1,27 @@
+import { getErrorMessage, type ErrorMessage } from "../lib/error-message.ts";
+import { stringify } from "../lib/string.ts";
+
+/**
+ * Asserts that a code path is unreachable, typically the `default` case of a
+ * `switch`/`case` statement over a union type.
+ *
+ * TypeScript only accepts `value` if its type has already been narrowed to
+ * `never`, i.e. all members of the union have been handled in the preceding
+ * cases. This turns an unhandled case into a compile-time error as soon as
+ * the union gains a new member, while still throwing at runtime if an
+ * unexpected value slips through.
+ *
+ * @param value - The value that should be of type `never`.
+ * @param errorMessage - The error message to throw.
+ */
+export const assertNever = (
+  value: never,
+  errorMessage?: ErrorMessage,
+): never => {
+  throw new TypeError(
+    getErrorMessage(
+      errorMessage,
+      () => `Unexpected value: ${stringify(value)}`,
+    ),
+  );
+};