diff --git a/.changeset/brave-foxes-swim.md b/.changeset/brave-foxes-swim.md
new file mode 100644
index 00000000..065aaf75
--- /dev/null
+++ b/.changeset/brave-foxes-swim.md
@@ -0,0 +1,5 @@
+---
+"ox": patch
+---
+
+Added `Ed25519.toX25519PublicKey` and `Ed25519.toX25519PrivateKey` for converting Ed25519 keys to X25519 keys. Useful for performing X25519 Diffie-Hellman key exchange using an Ed25519 signing key pair.
diff --git a/src/core/Ed25519.ts b/src/core/Ed25519.ts
index 7a9eebac..7fc69443 100644
--- a/src/core/Ed25519.ts
+++ b/src/core/Ed25519.ts
@@ -1,4 +1,8 @@
-import { ed25519 } from '@noble/curves/ed25519'
+import {
+  ed25519,
+  edwardsToMontgomeryPriv,
+  edwardsToMontgomeryPub,
+} from '@noble/curves/ed25519'
 import * as Bytes from './Bytes.js'
 import type * as Errors from './Errors.js'
 import * as Hex from './Hex.js'
@@ -235,3 +239,101 @@ export declare namespace verify {
 
   type ErrorType = Bytes.from.ErrorType | Errors.GlobalErrorType
 }
+
+/**
+ * Converts an Ed25519 public key to an X25519 public key.
+ *
+ * This is useful for performing X25519 Diffie-Hellman key exchange
+ * using an Ed25519 signing key pair.
+ *
+ * @example
+ * ```ts twoslash
+ * import { Ed25519, X25519 } from 'ox'
+ *
+ * const { privateKey, publicKey } = Ed25519.createKeyPair()
+ *
+ * const x25519PublicKey = Ed25519.toX25519PublicKey({ publicKey })
+ * ```
+ *
+ * @param options - The options.
+ * @returns The X25519 public key.
+ */
+export function toX25519PublicKey<as extends 'Hex' | 'Bytes' = 'Hex'>(
+  options: toX25519PublicKey.Options<as>,
+): toX25519PublicKey.ReturnType<as> {
+  const { as = 'Hex', publicKey } = options
+  const publicKeyBytes = Bytes.from(publicKey)
+  const x25519PublicKeyBytes = edwardsToMontgomeryPub(publicKeyBytes)
+  if (as === 'Hex') return Hex.fromBytes(x25519PublicKeyBytes) as never
+  return x25519PublicKeyBytes as never
+}
+
+export declare namespace toX25519PublicKey {
+  type Options<as extends 'Hex' | 'Bytes' = 'Hex'> = {
+    /**
+     * Format of the returned public key.
+     * @default 'Hex'
+     */
+    as?: as | 'Hex' | 'Bytes' | undefined
+    /** Ed25519 public key to convert. */
+    publicKey: Hex.Hex | Bytes.Bytes
+  }
+
+  type ReturnType<as extends 'Hex' | 'Bytes'> =
+    | (as extends 'Bytes' ? Bytes.Bytes : never)
+    | (as extends 'Hex' ? Hex.Hex : never)
+
+  type ErrorType =
+    | Bytes.from.ErrorType
+    | Hex.fromBytes.ErrorType
+    | Errors.GlobalErrorType
+}
+
+/**
+ * Converts an Ed25519 private key to an X25519 private key.
+ *
+ * This is useful for performing X25519 Diffie-Hellman key exchange
+ * using an Ed25519 signing key pair.
+ *
+ * @example
+ * ```ts twoslash
+ * import { Ed25519, X25519 } from 'ox'
+ *
+ * const { privateKey, publicKey } = Ed25519.createKeyPair()
+ *
+ * const x25519PrivateKey = Ed25519.toX25519PrivateKey({ privateKey })
+ * ```
+ *
+ * @param options - The options.
+ * @returns The X25519 private key.
+ */
+export function toX25519PrivateKey<as extends 'Hex' | 'Bytes' = 'Hex'>(
+  options: toX25519PrivateKey.Options<as>,
+): toX25519PrivateKey.ReturnType<as> {
+  const { as = 'Hex', privateKey } = options
+  const privateKeyBytes = Bytes.from(privateKey)
+  const x25519PrivateKeyBytes = edwardsToMontgomeryPriv(privateKeyBytes)
+  if (as === 'Hex') return Hex.fromBytes(x25519PrivateKeyBytes) as never
+  return x25519PrivateKeyBytes as never
+}
+
+export declare namespace toX25519PrivateKey {
+  type Options<as extends 'Hex' | 'Bytes' = 'Hex'> = {
+    /**
+     * Format of the returned private key.
+     * @default 'Hex'
+     */
+    as?: as | 'Hex' | 'Bytes' | undefined
+    /** Ed25519 private key to convert. */
+    privateKey: Hex.Hex | Bytes.Bytes
+  }
+
+  type ReturnType<as extends 'Hex' | 'Bytes'> =
+    | (as extends 'Bytes' ? Bytes.Bytes : never)
+    | (as extends 'Hex' ? Hex.Hex : never)
+
+  type ErrorType =
+    | Bytes.from.ErrorType
+    | Hex.fromBytes.ErrorType
+    | Errors.GlobalErrorType
+}
diff --git a/src/core/_test/Ed25519.test.ts b/src/core/_test/Ed25519.test.ts
index 4b6f7a15..7eaa9ee9 100644
--- a/src/core/_test/Ed25519.test.ts
+++ b/src/core/_test/Ed25519.test.ts
@@ -1,4 +1,4 @@
-import { Bytes, Ed25519, Hex } from 'ox'
+import { Bytes, Ed25519, Hex, X25519 } from 'ox'
 import { describe, expect, test } from 'vitest'
 
 describe('createKeyPair', () => {
@@ -325,6 +325,109 @@ describe('verify', () => {
   })
 })
 
+describe('toX25519PublicKey', () => {
+  test('default (Hex)', () => {
+    const { publicKey } = Ed25519.createKeyPair()
+    const x25519PublicKey = Ed25519.toX25519PublicKey({ publicKey })
+    expect(typeof x25519PublicKey).toBe('string')
+    expect(x25519PublicKey).toMatch(/^0x[0-9a-fA-F]{64}$/)
+  })
+
+  test('with as: Bytes', () => {
+    const { publicKey } = Ed25519.createKeyPair()
+    const x25519PublicKey = Ed25519.toX25519PublicKey({
+      publicKey,
+      as: 'Bytes',
+    })
+    expect(x25519PublicKey).toBeInstanceOf(Uint8Array)
+    expect(x25519PublicKey).toHaveLength(32)
+  })
+
+  test('with Bytes public key', () => {
+    const { publicKey } = Ed25519.createKeyPair({ as: 'Bytes' })
+    const x25519PublicKey = Ed25519.toX25519PublicKey({ publicKey })
+    expect(typeof x25519PublicKey).toBe('string')
+    expect(x25519PublicKey).toMatch(/^0x[0-9a-fA-F]{64}$/)
+  })
+
+  test('deterministic', () => {
+    const { publicKey } = Ed25519.createKeyPair()
+    const a = Ed25519.toX25519PublicKey({ publicKey })
+    const b = Ed25519.toX25519PublicKey({ publicKey })
+    expect(a).toBe(b)
+  })
+
+  test('different from ed25519 public key', () => {
+    const { publicKey } = Ed25519.createKeyPair()
+    const x25519PublicKey = Ed25519.toX25519PublicKey({ publicKey })
+    expect(x25519PublicKey).not.toBe(publicKey)
+  })
+})
+
+describe('toX25519PrivateKey', () => {
+  test('default (Hex)', () => {
+    const { privateKey } = Ed25519.createKeyPair()
+    const x25519PrivateKey = Ed25519.toX25519PrivateKey({ privateKey })
+    expect(typeof x25519PrivateKey).toBe('string')
+    expect(x25519PrivateKey).toMatch(/^0x[0-9a-fA-F]{64}$/)
+  })
+
+  test('with as: Bytes', () => {
+    const { privateKey } = Ed25519.createKeyPair()
+    const x25519PrivateKey = Ed25519.toX25519PrivateKey({
+      privateKey,
+      as: 'Bytes',
+    })
+    expect(x25519PrivateKey).toBeInstanceOf(Uint8Array)
+    expect(x25519PrivateKey).toHaveLength(32)
+  })
+
+  test('with Bytes private key', () => {
+    const { privateKey } = Ed25519.createKeyPair({ as: 'Bytes' })
+    const x25519PrivateKey = Ed25519.toX25519PrivateKey({ privateKey })
+    expect(typeof x25519PrivateKey).toBe('string')
+    expect(x25519PrivateKey).toMatch(/^0x[0-9a-fA-F]{64}$/)
+  })
+
+  test('deterministic', () => {
+    const { privateKey } = Ed25519.createKeyPair()
+    const a = Ed25519.toX25519PrivateKey({ privateKey })
+    const b = Ed25519.toX25519PrivateKey({ privateKey })
+    expect(a).toBe(b)
+  })
+})
+
+describe('toX25519 integration', () => {
+  test('converted keys produce valid X25519 shared secret', () => {
+    const alice = Ed25519.createKeyPair()
+    const bob = Ed25519.createKeyPair()
+
+    const aliceX25519Priv = Ed25519.toX25519PrivateKey({
+      privateKey: alice.privateKey,
+    })
+    const bobX25519Pub = Ed25519.toX25519PublicKey({
+      publicKey: bob.publicKey,
+    })
+    const bobX25519Priv = Ed25519.toX25519PrivateKey({
+      privateKey: bob.privateKey,
+    })
+    const aliceX25519Pub = Ed25519.toX25519PublicKey({
+      publicKey: alice.publicKey,
+    })
+
+    const sharedA = X25519.getSharedSecret({
+      privateKey: aliceX25519Priv,
+      publicKey: bobX25519Pub,
+    })
+    const sharedB = X25519.getSharedSecret({
+      privateKey: bobX25519Priv,
+      publicKey: aliceX25519Pub,
+    })
+
+    expect(sharedA).toBe(sharedB)
+  })
+})
+
 describe('noble export', () => {
   test('exports noble curves ed25519', () => {
     expect(Ed25519.noble).toBeDefined()