diff --git a/dlls/api-ms-win-core-io-l1-1-1/api-ms-win-core-io-l1-1-1.spec b/dlls/api-ms-win-core-io-l1-1-1/api-ms-win-core-io-l1-1-1.spec
index e05728ee277..73aae1aa9ac 100644
--- a/dlls/api-ms-win-core-io-l1-1-1/api-ms-win-core-io-l1-1-1.spec
+++ b/dlls/api-ms-win-core-io-l1-1-1/api-ms-win-core-io-l1-1-1.spec
@@ -1,6 +1,6 @@
 @ stdcall CancelIo(long) kernel32.CancelIo
 @ stdcall CancelIoEx(long ptr) kernel32.CancelIoEx
-@ stub CancelSynchronousIo
+@ stdcall CancelSynchronousIo(long) kernel32.CancelSynchronousIo
 @ stdcall CreateIoCompletionPort(long long long long) kernel32.CreateIoCompletionPort
 @ stdcall DeviceIoControl(long long ptr long ptr long ptr ptr) kernel32.DeviceIoControl
 @ stdcall GetOverlappedResult(long ptr ptr long) kernel32.GetOverlappedResult
diff --git a/dlls/kernel32/file.c b/dlls/kernel32/file.c
index e43829ea67e..ee43b10a36f 100644
--- a/dlls/kernel32/file.c
+++ b/dlls/kernel32/file.c
@@ -685,6 +685,25 @@ BOOL WINAPI CancelIo(HANDLE handle)
     return TRUE;
 }
 
+/***********************************************************************
+ *             CancelSynchronousIo                   (KERNEL32.@)
+ *
+ * Marks pending synchronous I/O operations issued by the specified thread as cancelled
+ *
+ * PARAMS
+ *  handle [I] handle to the thread whose I/O operations should be cancelled
+ *
+ * RETURNS
+ *  Success: TRUE.
+ *  Failure: FALSE, check GetLastError().
+ */
+BOOL WINAPI CancelSynchronousIo(HANDLE thread)
+{
+    FIXME("(%p): stub\n", thread);
+    SetLastError(ERROR_CALL_NOT_IMPLEMENTED);
+    return FALSE;
+}
+
 /***********************************************************************
  *           _hread   (KERNEL32.@)
  */
diff --git a/dlls/kernel32/kernel32.spec b/dlls/kernel32/kernel32.spec
index e0cb154dfff..dfc305b86e9 100644
--- a/dlls/kernel32/kernel32.spec
+++ b/dlls/kernel32/kernel32.spec
@@ -210,7 +210,7 @@
 @ stub CancelDeviceWakeupRequest
 @ stdcall CancelIo(long)
 @ stdcall CancelIoEx(long ptr)
-# @ stub CancelSynchronousIo
+@ stdcall CancelSynchronousIo(long)
 # @ stub CancelThreadpoolIo
 @ stdcall CancelTimerQueueTimer(ptr ptr)
 @ stdcall CancelWaitableTimer(long)
diff --git a/include/winbase.h b/include/winbase.h
index dd320c20999..25bc5936f2d 100644
--- a/include/winbase.h
+++ b/include/winbase.h
@@ -1694,6 +1694,7 @@ WINBASEAPI BOOL        WINAPI CallNamedPipeW(LPCWSTR,LPVOID,DWORD,LPVOID,DWORD,L
 #define                       CallNamedPipe WINELIB_NAME_AW(CallNamedPipe)
 WINBASEAPI BOOL        WINAPI CancelIo(HANDLE);
 WINBASEAPI BOOL        WINAPI CancelIoEx(HANDLE,LPOVERLAPPED);
+WINBASEAPI BOOL        WINAPI CancelSynchronousIo(HANDLE);
 WINBASEAPI BOOL        WINAPI CancelTimerQueueTimer(HANDLE,HANDLE);
 WINBASEAPI BOOL        WINAPI CancelWaitableTimer(HANDLE);
 WINBASEAPI BOOL        WINAPI CheckNameLegalDOS8Dot3A(const char*,char*,DWORD,BOOL*,BOOL*);