diff --git a/src/argon2/argon2-key-generator.ts b/src/argon2/argon2-key-generator.ts
index 24dd13e..31698df 100644
--- a/src/argon2/argon2-key-generator.ts
+++ b/src/argon2/argon2-key-generator.ts
@@ -13,11 +13,13 @@ export function createArgon2KeyGenerator({
   passes,
   parallelism,
   tagLength,
+  pepper,
 }: {
   memory: number;
   passes: number;
   parallelism: number;
   tagLength: number;
+  pepper?: BinaryLike | undefined;
 }): KeyGenerator {
   return {
     generateKey(password, salt) {
@@ -28,6 +30,7 @@ export function createArgon2KeyGenerator({
         passes,
         parallelism,
         tagLength,
+        secret: pepper,
       });
     },
   };
diff --git a/src/argon2/argon2.ts b/src/argon2/argon2.ts
index 3b70189..ace2336 100644
--- a/src/argon2/argon2.ts
+++ b/src/argon2/argon2.ts
@@ -16,6 +16,7 @@ const optionsSchema = z
     parallelism: z.number().min(1).max(MAX_UINT24),
     tagLength: z.number().min(4).max(MAX_UINT32),
     saltLength: z.number().min(16).max(1024),
+    pepper: z.string().min(1).max(1024).optional(),
   })
   .refine(
     (params) => {
@@ -82,6 +83,7 @@ export function createArgon2Hashing(
           passes: phcNode.params.passes,
           parallelism: phcNode.params.parallelism,
           tagLength: phcNode.hash.byteLength,
+          pepper: defaultOptions.pepper,
         }).generateKey(password, phcNode.salt);
 
         return timingSafeEqual(targetKey, phcNode.hash);
diff --git a/src/argon2/pepper.test.ts b/src/argon2/pepper.test.ts
new file mode 100644
index 0000000..9241769
--- /dev/null
+++ b/src/argon2/pepper.test.ts
@@ -0,0 +1,66 @@
+import { createArgon2Hashing } from "./argon2.ts";
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+
+void describe("Argon2 Pepper Support", () => {
+  const password = "my-secret-password";
+  const firstPepper = "secret-pepper-one";
+  const secondPepper = "secret-pepper-two";
+
+  void it("should successfully verify a password when the correct pepper is provided", async () => {
+    const hashing = createArgon2Hashing({ pepper: firstPepper });
+    const hash = await hashing.hash(password);
+
+    const isValid = await hashing.verify(password, hash);
+    assert.strictEqual(
+      isValid,
+      true,
+      "Verification with the correct pepper should succeed",
+    );
+  });
+
+  void it("should fail verification if a different pepper is used", async () => {
+    const hashingA = createArgon2Hashing({ pepper: firstPepper });
+    const hashingB = createArgon2Hashing({ pepper: secondPepper });
+    const hash = await hashingA.hash(password);
+    const isValid = await hashingB.verify(password, hash);
+
+    assert.strictEqual(
+      isValid,
+      false,
+      "Verification with a mismatched pepper should fail",
+    );
+  });
+});
+
+void describe("Argon2 Pepper Validation", () => {
+  const minIncludedPepper = "a";
+  const minExcludedPepper = "";
+
+  const maxIncludedPepper = "a".repeat(1024);
+  const maxExcludedPepper = "a".repeat(1025);
+
+  for (const pepper of [minIncludedPepper, maxIncludedPepper]) {
+    const password = "my-secret-password";
+
+    void it(`should verify a password when the pepper is ${pepper.length} characters long`, async () => {
+      const hashing = createArgon2Hashing({ pepper });
+      const hash = await hashing.hash(password);
+
+      const isValid = await hashing.verify(password, hash);
+      assert.strictEqual(
+        isValid,
+        true,
+        "Verification with the correct pepper should succeed",
+      );
+    });
+  }
+
+  for (const pepper of [minExcludedPepper, maxExcludedPepper]) {
+    void it(`should throw an error if the pepper is ${pepper.length} characters long`, () => {
+      assert.throws(() => {
+        createArgon2Hashing({ pepper });
+      }, "Expected error to be thrown");
+    });
+  }
+});