diff --git a/build.cake b/build.cake
index cf1a39c..527ed51 100644
--- a/build.cake
+++ b/build.cake
@@ -81,7 +81,7 @@ Task("Build")
 
 const string DOORSTOP_VER_WIN = "3.4.0.0";
 const string DOORSTOP_VER_UNIX = "1.5.1.0";
-const string MONO_VER = "2021.5.16";
+const string MONO_VER = "2021.6.20";
 const string DOORSTOP_DLL = "winhttp.dll";
 Task("DownloadDependencies")
     .Does(() =>