diff --git a/libc/bionic/ndk_cruft.cpp b/libc/bionic/ndk_cruft.cpp
index 46cc1f0a4..4900a8a9a 100644
--- a/libc/bionic/ndk_cruft.cpp
+++ b/libc/bionic/ndk_cruft.cpp
@@ -222,4 +222,9 @@ extern "C" int __futex_wait(volatile void* ftx, int value, const struct timespec
   return __futex(ftx, FUTEX_WAIT, value, timeout);
 }
 
+// Unity's libmono uses this.
+extern "C" int tkill(pid_t tid, int sig) {
+  return syscall(__NR_tkill, tid, sig);
+}
+
 #endif