diff --git a/ChangeLog.md b/ChangeLog.md
index 455dc79c20223..6af38160f9ce1 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -17,6 +17,8 @@ See docs/process.md for how version tagging works.
 
 Current Trunk
 -------------
+- Add `getentropy` in `sys/random.h`, and use that from libc++'s
+  `random_device`. This is more efficient, see #12240.
 
 2.0.4: 09/16/2020
 -----------------
diff --git a/src/library.js b/src/library.js
index e2b42e443d387..1651d36d129cb 100644
--- a/src/library.js
+++ b/src/library.js
@@ -2845,6 +2845,47 @@ LibraryManager.library = {
   endgrent: function() { throw 'endgrent: TODO' },
   setgrent: function() { throw 'setgrent: TODO' },
 
+  // random.h
+
+  // TODO: consider allowing the API to get a parameter for the number of
+  // bytes.
+  $getRandomDevice: function() {
+    if (typeof crypto === 'object' && typeof crypto['getRandomValues'] === 'function') {
+      // for modern web browsers
+      var randomBuffer = new Uint8Array(1);
+      return function() { crypto.getRandomValues(randomBuffer); return randomBuffer[0]; };
+    } else
+#if ENVIRONMENT_MAY_BE_NODE
+    if (ENVIRONMENT_IS_NODE) {
+      // for nodejs with or without crypto support included
+      try {
+        var crypto_module = require('crypto');
+        // nodejs has crypto support
+        return function() { return crypto_module['randomBytes'](1)[0]; };
+      } catch (e) {
+        // nodejs doesn't have crypto support
+      }
+    }
+#endif // ENVIRONMENT_MAY_BE_NODE
+    // we couldn't find a proper implementation, as Math.random() is not suitable for /dev/random, see emscripten-core/emscripten/pull/7096
+#if ASSERTIONS
+    return function() { abort("no cryptographic support found for randomDevice. consider polyfilling it if you want to use something insecure like Math.random(), e.g. put this in a --pre-js: var crypto = { getRandomValues: function(array) { for (var i = 0; i < array.length; i++) array[i] = (Math.random()*256)|0 } };"); };
+#else
+    return function() { abort("randomDevice"); };
+#endif
+  },
+
+  getentropy__deps: ['$getRandomDevice'],
+  getentropy: function(buffer, size) {
+    if (!_getentropy.randomDevice) {
+      _getentropy.randomDevice = getRandomDevice();
+    }
+    for (var i = 0; i < size; i++) {
+      {{{ makeSetValue('buffer', 'i', '_getentropy.randomDevice()', 'i8') }}}
+    }
+    return 0;
+  },
+
   // ==========================================================================
   // emscripten.h
   // ==========================================================================
diff --git a/src/library_fs.js b/src/library_fs.js
index 528bbb3a2c1d9..ee4d55d4d591f 100644
--- a/src/library_fs.js
+++ b/src/library_fs.js
@@ -5,7 +5,7 @@
  */
 
 mergeInto(LibraryManager.library, {
-  $FS__deps: ['$setErrNo', '$PATH', '$PATH_FS', '$TTY', '$MEMFS',
+  $FS__deps: ['$setErrNo', '$getRandomDevice', '$PATH', '$PATH_FS', '$TTY', '$MEMFS',
 #if LibraryManager.has('library_idbfs.js')
     '$IDBFS',
 #endif
@@ -1344,33 +1344,7 @@ FS.staticInit();` +
       FS.mkdev('/dev/tty', FS.makedev(5, 0));
       FS.mkdev('/dev/tty1', FS.makedev(6, 0));
       // setup /dev/[u]random
-      var random_device;
-      if (typeof crypto === 'object' && typeof crypto['getRandomValues'] === 'function') {
-        // for modern web browsers
-        var randomBuffer = new Uint8Array(1);
-        random_device = function() { crypto.getRandomValues(randomBuffer); return randomBuffer[0]; };
-      } else
-#if ENVIRONMENT_MAY_BE_NODE
-      if (ENVIRONMENT_IS_NODE) {
-        // for nodejs with or without crypto support included
-        try {
-          var crypto_module = require('crypto');
-          // nodejs has crypto support
-          random_device = function() { return crypto_module['randomBytes'](1)[0]; };
-        } catch (e) {
-          // nodejs doesn't have crypto support
-        }
-      } else
-#endif // ENVIRONMENT_MAY_BE_NODE
-      {}
-      if (!random_device) {
-        // we couldn't find a proper implementation, as Math.random() is not suitable for /dev/random, see emscripten-core/emscripten/pull/7096
-#if ASSERTIONS
-        random_device = function() { abort("no cryptographic support found for random_device. consider polyfilling it if you want to use something insecure like Math.random(), e.g. put this in a --pre-js: var crypto = { getRandomValues: function(array) { for (var i = 0; i < array.length; i++) array[i] = (Math.random()*256)|0 } };"); };
-#else
-        random_device = function() { abort("random_device"); };
-#endif
-      }
+      var random_device = getRandomDevice();
       FS.createDevice('/dev', 'random', random_device);
       FS.createDevice('/dev', 'urandom', random_device);
       // we're not going to emulate the actual shm device,
diff --git a/system/include/compat/sys/random.h b/system/include/compat/sys/random.h
new file mode 100644
index 0000000000000..72f3bf7a26bd6
--- /dev/null
+++ b/system/include/compat/sys/random.h
@@ -0,0 +1,14 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// This is used by libc++ as an efficient way to get high-quality random data
+// (more efficiently than via the filesystem using /dev/urandom).
+// Upstream musl added support for this, so we can switch to that, but it isn't
+// where libc++ looks for it (which is here and not unistd.h), and it uses a
+// syscall which is unnecessary indirection for us.
+int getentropy(void *buffer, size_t length);
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/system/include/libcxx/__config b/system/include/libcxx/__config
index 088213618b1e7..34f4d7731fdba 100644
--- a/system/include/libcxx/__config
+++ b/system/include/libcxx/__config
@@ -308,7 +308,7 @@
    // random data even when using sandboxing mechanisms such as chroots,
    // Capsicum, etc.
 #  define _LIBCPP_USING_ARC4_RANDOM
-#elif defined(__Fuchsia__) || defined(__wasi__)
+#elif defined(__Fuchsia__) || defined(__wasi__) || defined(__EMSCRIPTEN__)
 #  define _LIBCPP_USING_GETENTROPY
 #elif defined(__native_client__)
    // NaCl's sandbox (which PNaCl also runs in) doesn't allow filesystem access,
diff --git a/system/lib/libcxx/readme.txt b/system/lib/libcxx/readme.txt
index 969e0884e5ae3..acd017aff5b98 100755
--- a/system/lib/libcxx/readme.txt
+++ b/system/lib/libcxx/readme.txt
@@ -20,3 +20,5 @@ Local modifications are marked with the comment: 'XXX EMSCRIPTEN'
 3. Define _LIBCPP_ELAST in libcxx/include/config_elast.h
 
 4. Set init_priority of __start_std_streams in libcxx/iostream.cpp
+
+5. Use _LIBCPP_USING_GETENTROPY (like wasi)
diff --git a/tests/core/test_random_device.cpp b/tests/core/test_random_device.cpp
index 9ab6d3c7f650a..4c8de8efe83c0 100644
--- a/tests/core/test_random_device.cpp
+++ b/tests/core/test_random_device.cpp
@@ -12,7 +12,7 @@ auto main()
 try
 {
   std::random_device rd;
-  std::cout << "random was read" << "\n";
+  std::cout << rd() << ", a random was read\n";
 }
 catch( const std::exception& e )
 {