diff --git a/emcc.py b/emcc.py
index 549eea562a647..97b615b4866ac 100755
--- a/emcc.py
+++ b/emcc.py
@@ -2326,6 +2326,7 @@ def phase_linker_setup(options, state, newargs):
         '_wasmfs_read_file',
         '_wasmfs_write_file',
         '_wasmfs_open',
+        '_wasmfs_allocate',
         '_wasmfs_close',
         '_wasmfs_write',
         '_wasmfs_pwrite',
diff --git a/src/library_wasmfs.js b/src/library_wasmfs.js
index 48bb8cf00557a..5c4df6481236d 100644
--- a/src/library_wasmfs.js
+++ b/src/library_wasmfs.js
@@ -189,7 +189,9 @@ FS.createPreloadedFile = FS_createPreloadedFile;
 
       return bytesRead;
     },
-    // TODO: allocate
+    allocate: (stream, offset, length) => {
+      return FS.handleError(__wasmfs_allocate(stream.fd, {{{ splitI64('offset') }}}, {{{ splitI64('length') }}}));
+    },
     // TODO: mmap
     // TODO: msync
     // TODO: munmap
diff --git a/system/lib/libc/musl/arch/emscripten/syscall_arch.h b/system/lib/libc/musl/arch/emscripten/syscall_arch.h
index bad86ad4e98df..89dcfeadc7188 100644
--- a/system/lib/libc/musl/arch/emscripten/syscall_arch.h
+++ b/system/lib/libc/musl/arch/emscripten/syscall_arch.h
@@ -96,7 +96,7 @@ int __syscall_fchmodat(int dirfd, intptr_t path, int mode, ...);
 int __syscall_faccessat(int dirfd, intptr_t path, int amode, int flags);
 int __syscall_pselect6(int nfds, intptr_t readfds, intptr_t writefds, intptr_t exceptfds, intptr_t timeout, intptr_t sigmaks);
 int __syscall_utimensat(int dirfd, intptr_t path, intptr_t times, int flags);
-int __syscall_fallocate(int fd, int mode, uint64_t off, uint64_t len);
+int __syscall_fallocate(int fd, int mode, int64_t off, int64_t len);
 int __syscall_dup3(int fd, int suggestfd, int flags);
 int __syscall_pipe2(intptr_t fds, int flags);
 int __syscall_recvmmsg(int sockfd, intptr_t msgvec, size_t vlen, int flags, ...);
diff --git a/system/lib/wasmfs/js_api.cpp b/system/lib/wasmfs/js_api.cpp
index 8c58589f234c4..56f9229c2576c 100644
--- a/system/lib/wasmfs/js_api.cpp
+++ b/system/lib/wasmfs/js_api.cpp
@@ -129,6 +129,10 @@ int _wasmfs_open(char* path, int flags, mode_t mode) {
   return __syscall_openat(AT_FDCWD, (intptr_t)path, flags, mode);
 }
 
+int _wasmfs_allocate(int fd, int64_t off, int64_t len) {
+  return __syscall_fallocate(fd, 0, off, len);
+}
+
 int _wasmfs_mknod(char* path, mode_t mode, dev_t dev) {
   return __syscall_mknodat(AT_FDCWD, (intptr_t)path, mode, dev);
 }
diff --git a/system/lib/wasmfs/syscalls.cpp b/system/lib/wasmfs/syscalls.cpp
index 9eb1e0fb5c33d..0e581d7dabe93 100644
--- a/system/lib/wasmfs/syscalls.cpp
+++ b/system/lib/wasmfs/syscalls.cpp
@@ -1358,7 +1358,7 @@ int __syscall_poll(intptr_t fds_, int nfds, int timeout) {
   return nonzero;
 }
 
-int __syscall_fallocate(int fd, int mode, uint64_t off, uint64_t len) {
+int __syscall_fallocate(int fd, int mode, int64_t off, int64_t len) {
   assert(mode == 0); // TODO, but other modes were never supported in the old FS
 
   auto fileTable = wasmFS.getFileTable().locked();
diff --git a/test/fs/test_fs_js_api.c b/test/fs/test_fs_js_api.c
index dae0daa0875ad..5c6338a7dc485 100644
--- a/test/fs/test_fs_js_api.c
+++ b/test/fs/test_fs_js_api.c
@@ -4,6 +4,46 @@
 #include <assert.h>
 #include <fcntl.h>
 
+void test_fs_allocate() {
+    EM_ASM(
+        FS.writeFile("allocatetestfile", 'a=1\nb=2\n');
+    );
+    struct stat allocateStat;
+    stat("allocatetestfile", &allocateStat);
+    assert(allocateStat.st_size == 8);
+
+    EM_ASM(
+        // Allocate more space at the very end.
+        var stream = FS.open("allocatetestfile", "w");
+        FS.allocate(stream, 8, 10);
+    );
+    stat("allocatetestfile", &allocateStat);
+    assert(allocateStat.st_size == 18);
+
+    EM_ASM(
+        // Reduce allocated space at the very start.
+        var stream = FS.open("allocatetestfile", "w");
+        FS.allocate(stream, 0, 4);
+    );
+    stat("allocatetestfile", &allocateStat);
+    assert(allocateStat.st_size == 4);
+
+    EM_ASM(
+        var stream = FS.open("allocatetestfile", "w");
+        
+        var ex;
+        try {
+            // Attempt to allocate negative length.
+            FS.allocate(stream, 0, -1);
+        } catch (err) {
+            ex = err;
+        }
+        assert(ex.name === "ErrnoError" && ex.errno === 28 /* EINVAL */);
+    );
+
+    remove("allocatetestfile");
+}
+
 void test_fs_truncate() {
     EM_ASM(
         FS.writeFile('truncatetest', 'a=1\nb=2\n');
@@ -182,12 +222,11 @@ int main() {
         FS.mkdir('/dir2');
     );
 
-    struct stat s;
-    stat("/dir1", &s);
-    assert(S_ISDIR(s.st_mode));
-    stat("/dir2", &s);
-    assert(S_ISDIR(s.st_mode));
-
+    struct stat rmdirStat;
+    stat("/dir1", &rmdirStat);
+    assert(S_ISDIR(rmdirStat.st_mode));
+    stat("/dir2", &rmdirStat);
+    assert(S_ISDIR(rmdirStat.st_mode));
 
     EM_ASM(
         // Remove the multiple directories
@@ -255,15 +294,17 @@ int main() {
 
         FS.create("createtest", 0400); /* S_IRUSR */
     );
-    struct stat stats;
-    stat("mknodtest", &stats);
+    struct stat mknodStats;
+    stat("mknodtest", &mknodStats);
+
+    assert(S_ISREG(mknodStats.st_mode));
+    assert(mknodStats.st_mode & 0777);
 
-    assert(S_ISREG(stats.st_mode));
-    assert(stats.st_mode & 0777);
+    stat("createtest", &mknodStats);
+    assert(S_ISREG(mknodStats.st_mode));
+    assert(mknodStats.st_mode & 0400);
 
-    stat("createtest", &stats);
-    assert(S_ISREG(stats.st_mode));
-    assert(stats.st_mode & 0400);
+    test_fs_allocate();
 
     test_fs_truncate();