Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -4737,6 +4737,8 @@
 def _SLASH_Tp : CLCompileJoinedOrSeparate<"Tp">,
   HelpText<"Treat <file> as C++ source file">, MetaVarName<"<file>">;
 def _SLASH_TP : CLCompileFlag<"TP">, HelpText<"Treat all source files as C++">;
+def _SLASH_vctoolsdir : CLJoinedOrSeparate<"vctoolsdir">,
+  HelpText<"Path to the VCToolChain">, MetaVarName<"<dir>">;
 def _SLASH_volatile_iso : Option<["/", "-"], "volatile:iso", KIND_FLAG>,
   Group<_SLASH_volatile_Group>, Flags<[CLOption, DriverOption]>,
   HelpText<"Volatile loads and stores have standard semantics">;
Index: clang/lib/Driver/ToolChains/MSVC.cpp
===================================================================
--- clang/lib/Driver/ToolChains/MSVC.cpp
+++ clang/lib/Driver/ToolChains/MSVC.cpp
@@ -66,6 +66,20 @@
 static bool getSystemRegistryString(const char *keyPath, const char *valueName,
                                     std::string &value, std::string *phValue);
 
+// Check command line arguments to try and find a toolchain.
+static bool
+findVCToolChainViaCommandLine(const ArgList &Args, std::string &Path,
+                              MSVCToolChain::ToolsetLayout &VSLayout) {
+  // Don't validate the input; trust the value supplied by the user.
+  // The primary motivation is to prevent unnecessary file and registry access.
+  if (Arg *A = Args.getLastArg(options::OPT__SLASH_vctoolsdir)) {
+    Path = A->getValue();
+    VSLayout = MSVCToolChain::ToolsetLayout::VS2017OrNewer;
+    return true;
+  }
+  return false;
+}
+
 // Check various environment variables to try and find a toolchain.
 static bool findVCToolChainViaEnvironment(std::string &Path,
                                           MSVCToolChain::ToolsetLayout &VSLayout) {
@@ -747,11 +761,12 @@
   if (getDriver().getInstalledDir() != getDriver().Dir)
     getProgramPaths().push_back(getDriver().Dir);
 
-  // Check the environment first, since that's probably the user telling us
-  // what they want to use.
-  // Failing that, just try to find the newest Visual Studio version we can
-  // and use its default VC toolchain.
-  findVCToolChainViaEnvironment(VCToolChainPath, VSLayout) ||
+  // Check the command line first, that's the user explicitly telling us what to
+  // use. Check the environment next, in case we're being invoked from a VS
+  // command prompt. Failing that, just try to find the newest Visual Studio
+  // version we can and use its default VC toolchain.
+  findVCToolChainViaCommandLine(Args, VCToolChainPath, VSLayout) ||
+      findVCToolChainViaEnvironment(VCToolChainPath, VSLayout) ||
       findVCToolChainViaSetupConfig(VCToolChainPath, VSLayout) ||
       findVCToolChainViaRegistry(VCToolChainPath, VSLayout);
 }
Index: clang/test/Driver/cl-options.c
===================================================================
--- clang/test/Driver/cl-options.c
+++ clang/test/Driver/cl-options.c
@@ -682,4 +682,7 @@
 // CLANG-NOT: "--dependent-lib=libcmt"
 // CLANG-NOT: "-vectorize-slp"
 
+// RUN: %clang_cl -vctoolsdir "" -### -- %s 2>&1 | FileCheck %s --check-prefix VCTOOLSDIR
+// VCTOOLSDIR: "-cc1" "-triple" "x86_64-pc-windows-msvc19.11.0"
+
 void f() { }