diff --git a/lib/index.js b/lib/index.js
index 06f07e1..f227930 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -121,7 +121,7 @@ const changelog =
 const emptyPlugins = []
 /** @type {Readonly<RemarkRehypeOptions>} */
 const emptyRemarkRehypeOptions = {allowDangerousHtml: true}
-const safeProtocol = /^(https?|ircs?|mailto|xmpp)$/i
+const defaultSafeProtocol = /^(https?|ircs?|mailto|xmpp)$/i
 
 // Mutable because we `delete` any time it’s used and a message is sent.
 /** @type {ReadonlyArray<Readonly<Deprecation>>} */
@@ -317,7 +317,7 @@ function post(tree, options) {
   const disallowedElements = options.disallowedElements
   const skipHtml = options.skipHtml
   const unwrapDisallowed = options.unwrapDisallowed
-  const urlTransform = options.urlTransform || defaultUrlTransform
+  const urlTransform = options.urlTransform || ((v) => defaultUrlTransform(v))
 
   for (const deprecation of deprecations) {
     if (Object.hasOwn(options, deprecation.from)) {
@@ -413,16 +413,19 @@ function post(tree, options) {
  * Make a URL safe.
  *
  * This follows how GitHub works.
- * It allows the protocols `http`, `https`, `irc`, `ircs`, `mailto`, and `xmpp`,
- * and URLs relative to the current protocol (such as `/something`).
+ * By default it allows the protocols `http`, `https`, `irc`, `ircs`, `mailto`,
+ * and `xmpp`, and URLs relative to the current protocol (such as `/something`);
+ * pass `safeProtocol` to override which protocols are allowed.
  *
  * @satisfies {UrlTransform}
  * @param {string} value
  *   URL.
+ * @param {RegExp} [safeProtocol]
+ *   Regex matching protocols to allow (default: `/^(https?|ircs?|mailto|xmpp)$/i`);
  * @returns {string}
  *   Safe URL.
  */
-export function defaultUrlTransform(value) {
+export function defaultUrlTransform(value, safeProtocol = defaultSafeProtocol) {
   // Same as:
   // <https://github.com/micromark/micromark/blob/929275e/packages/micromark-util-sanitize-uri/dev/index.js#L34>
   // But without the `encode` part.
diff --git a/readme.md b/readme.md
index 949b180..0739514 100644
--- a/readme.md
+++ b/readme.md
@@ -34,7 +34,7 @@ React component to render markdown.
   * [`Markdown`](#markdown)
   * [`MarkdownAsync`](#markdownasync)
   * [`MarkdownHooks`](#markdownhooks)
-  * [`defaultUrlTransform(url)`](#defaulturltransformurl)
+  * [`defaultUrlTransform(url, safeProtocol)`](#defaulturltransformurl-safeprotocol)
   * [`AllowElement`](#allowelement)
   * [`Components`](#components)
   * [`ExtraProps`](#extraprops)
@@ -238,18 +238,21 @@ see [`MarkdownAsync`][api-markdown-async].
 
 React node (`ReactNode`).
 
-### `defaultUrlTransform(url)`
+### `defaultUrlTransform(url, safeProtocol)`
 
 Make a URL safe.
 
 This follows how GitHub works.
-It allows the protocols `http`, `https`, `irc`, `ircs`, `mailto`, and `xmpp`,
-and URLs relative to the current protocol (such as `/something`).
+By default it allows the protocols `http`, `https`, `irc`, `ircs`, `mailto`,
+and `xmpp`, and URLs relative to the current protocol (such as `/something`);
+pass `safeProtocol` to override which protocols are allowed.
 
 ###### Parameters
 
 * `url` (`string`)
   — URL
+* `safeProtocol` (`RegExp`, optional)
+  — regex matching protocols to allow (default: `/^(https?|ircs?|mailto|xmpp)$/i`)
 
 ###### Returns
 
@@ -829,7 +832,7 @@ abide by its terms.
 
 [api-components]: #components
 
-[api-default-url-transform]: #defaulturltransformurl
+[api-default-url-transform]: #defaulturltransformurl-safeprotocol
 
 [api-extra-props]: #extraprops
 
diff --git a/test.jsx b/test.jsx
index 1b254db..3383bec 100644
--- a/test.jsx
+++ b/test.jsx
@@ -24,7 +24,11 @@ import {render, waitFor} from '@testing-library/react'
 import concatStream from 'concat-stream'
 import {Component} from 'react'
 import {renderToPipeableStream, renderToStaticMarkup} from 'react-dom/server'
-import Markdown, {MarkdownAsync, MarkdownHooks} from 'react-markdown'
+import Markdown, {
+  MarkdownAsync,
+  MarkdownHooks,
+  defaultUrlTransform
+} from 'react-markdown'
 import rehypeRaw from 'rehype-raw'
 import rehypeStarryNight from 'rehype-starry-night'
 import remarkGfm from 'remark-gfm'
@@ -414,6 +418,40 @@ test('Markdown', async function (t) {
     )
   })
 
+  await t.test(
+    'should support a custom `safeProtocol` via `urlTransform`',
+    function () {
+      assert.equal(
+        renderToStaticMarkup(
+          <Markdown
+            children="[a](tel:+123)"
+            urlTransform={function (url) {
+              return defaultUrlTransform(url, /^tel$/i)
+            }}
+          />
+        ),
+        '<p><a href="tel:+123">a</a></p>'
+      )
+    }
+  )
+
+  await t.test(
+    'should drop URLs whose protocol is not in `safeProtocol`',
+    function () {
+      assert.equal(
+        renderToStaticMarkup(
+          <Markdown
+            children="[a](https://b.com)"
+            urlTransform={function (url) {
+              return defaultUrlTransform(url, /^tel$/i)
+            }}
+          />
+        ),
+        '<p><a href="">a</a></p>'
+      )
+    }
+  )
+
   await t.test('should support `skipHtml`', function () {
     const actual = renderToStaticMarkup(
       <Markdown children="a<i>b</i>c" skipHtml />
@@ -1058,6 +1096,37 @@ test('Markdown', async function (t) {
   })
 })
 
+test('defaultUrlTransform', async function (t) {
+  await t.test('should allow default protocols', function () {
+    assert.equal(
+      defaultUrlTransform('https://example.com'),
+      'https://example.com'
+    )
+    assert.equal(defaultUrlTransform('mailto:a@b.c'), 'mailto:a@b.c')
+  })
+
+  await t.test('should drop unsafe protocols by default', function () {
+    assert.equal(defaultUrlTransform('data:text/html,<script>'), '')
+  })
+
+  await t.test('should allow relative URLs', function () {
+    assert.equal(defaultUrlTransform('/a/b'), '/a/b')
+    assert.equal(defaultUrlTransform('a?x:y'), 'a?x:y')
+    assert.equal(defaultUrlTransform('a#x:y'), 'a#x:y')
+  })
+
+  await t.test('should accept a custom `safeProtocol`', function () {
+    assert.equal(defaultUrlTransform('tel:+123', /^tel$/i), 'tel:+123')
+  })
+
+  await t.test(
+    'should drop protocols not matched by `safeProtocol`',
+    function () {
+      assert.equal(defaultUrlTransform('https://example.com', /^tel$/i), '')
+    }
+  )
+})
+
 test('MarkdownAsync', async function (t) {
   await t.test('should support `MarkdownAsync` (1)', async function () {
     assert.throws(function () {