diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -4746,6 +4746,8 @@
 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">;
diff --git a/clang/lib/Driver/ToolChains/MSVC.cpp b/clang/lib/Driver/ToolChains/MSVC.cpp
--- a/clang/lib/Driver/ToolChains/MSVC.cpp
+++ b/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);
 }
@@ -1263,15 +1278,18 @@
     return;
 
   // Honor %INCLUDE%. It should know essential search paths with vcvarsall.bat.
-  if (llvm::Optional<std::string> cl_include_dir =
-          llvm::sys::Process::GetEnv("INCLUDE")) {
-    SmallVector<StringRef, 8> Dirs;
-    StringRef(*cl_include_dir)
-        .split(Dirs, ";", /*MaxSplit=*/-1, /*KeepEmpty=*/false);
-    for (StringRef Dir : Dirs)
-      addSystemInclude(DriverArgs, CC1Args, Dir);
-    if (!Dirs.empty())
-      return;
+  // Skip if the user expressly set a vctoolsdir
+  if (!DriverArgs.getLastArg(options::OPT__SLASH_vctoolsdir)) {
+    if (llvm::Optional<std::string> cl_include_dir =
+            llvm::sys::Process::GetEnv("INCLUDE")) {
+      SmallVector<StringRef, 8> Dirs;
+      StringRef(*cl_include_dir)
+          .split(Dirs, ";", /*MaxSplit=*/-1, /*KeepEmpty=*/false);
+      for (StringRef Dir : Dirs)
+        addSystemInclude(DriverArgs, CC1Args, Dir);
+      if (!Dirs.empty())
+        return;
+    }
   }
 
   // When built with access to the proper Windows APIs, try to actually find
diff --git a/clang/test/Driver/cl-options.c b/clang/test/Driver/cl-options.c
--- a/clang/test/Driver/cl-options.c
+++ b/clang/test/Driver/cl-options.c
@@ -682,4 +682,13 @@
 // CLANG-NOT: "--dependent-lib=libcmt"
 // CLANG-NOT: "-vectorize-slp"
 
+// Validate that the default triple is used when run an empty tools dir is specified
+// RUN: %clang_cl -vctoolsdir "" -### -- %s 2>&1 | FileCheck %s --check-prefix VCTOOLSDIR
+// VCTOOLSDIR: "-cc1" "-triple" "x86_64-pc-windows-msvc19.11.0"
+
+// Validate that built-in include paths are based on the supplied path
+// RUN: %clang_cl -vctoolsdir "/fake" -### -- %s 2>&1 | FileCheck %s --check-prefix FAKEDIR
+// FAKEDIR: "-internal-isystem" "/fake{{.}}include"
+// FAKEDIR: "-internal-isystem" "/fake{{.}}atlmfc{{.}}include"
+
 void f() { }