diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -94,6 +94,7 @@
 #include "llvm/Support/StringSaver.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
+#include <cstdlib> // ::getenv
 #include <map>
 #include <memory>
 #include <utility>
@@ -1066,6 +1067,12 @@
 }
 
 bool Driver::loadDefaultConfigFiles(ArrayRef<StringRef> CfgFileSearchDirs) {
+  // Disable default config if CLANG_NO_DEFAULT_CONFIG is set to a non-empty
+  // value.
+  if (const char *NoConfigEnv = ::getenv("CLANG_NO_DEFAULT_CONFIG")) {
+    if (*NoConfigEnv)
+      return false;
+  }
   if (CLOptions && CLOptions->hasArg(options::OPT_no_default_config))
     return false;
 
diff --git a/clang/test/Driver/config-file3.c b/clang/test/Driver/config-file3.c
--- a/clang/test/Driver/config-file3.c
+++ b/clang/test/Driver/config-file3.c
@@ -1,6 +1,7 @@
 // REQUIRES: shell
 // REQUIRES: x86-registered-target
 
+// RUN: unset CLANG_NO_DEFAULT_CONFIG
 // RUN: rm -rf %t && mkdir %t
 
 //--- If config file is specified by relative path (workdir/cfg-s2), it is searched for by that path.
diff --git a/clang/test/Driver/env.c b/clang/test/Driver/env.c
--- a/clang/test/Driver/env.c
+++ b/clang/test/Driver/env.c
@@ -4,13 +4,13 @@
 //
 // REQUIRES: shell
 // The PATH variable is heavily used when trying to find a linker.
-// RUN: env -i LC_ALL=C LD_LIBRARY_PATH="$LD_LIBRARY_PATH" \
+// RUN: env -i LC_ALL=C LD_LIBRARY_PATH="$LD_LIBRARY_PATH" CLANG_NO_DEFAULT_CONFIG=1 \
 // RUN:   %clang %s -### -o %t.o --target=i386-unknown-linux \
 // RUN:     --sysroot=%S/Inputs/basic_linux_tree \
 // RUN:     --rtlib=platform -no-pie \
 // RUN:     --gcc-toolchain="" 2>&1 | FileCheck --check-prefix=CHECK-LD-32 %s
 //
-// RUN: env -i LC_ALL=C PATH="" LD_LIBRARY_PATH="$LD_LIBRARY_PATH" \
+// RUN: env -i LC_ALL=C PATH="" LD_LIBRARY_PATH="$LD_LIBRARY_PATH" CLANG_NO_DEFAULT_CONFIG=1 \
 // RUN:   %clang %s -### -o %t.o --target=i386-unknown-linux \
 // RUN:     --sysroot=%S/Inputs/basic_linux_tree \
 // RUN:     --rtlib=platform -no-pie \
diff --git a/clang/test/Unit/lit.cfg.py b/clang/test/Unit/lit.cfg.py
--- a/clang/test/Unit/lit.cfg.py
+++ b/clang/test/Unit/lit.cfg.py
@@ -71,3 +71,8 @@
 else:
     lit_config.warning("unable to inject shared library path on '{}'"
                        .format(platform.system()))
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
diff --git a/clang/test/lit.cfg.py b/clang/test/lit.cfg.py
--- a/clang/test/lit.cfg.py
+++ b/clang/test/lit.cfg.py
@@ -284,3 +284,8 @@
 
 if 'system-aix' in config.available_features:
         config.substitutions.append(('llvm-nm', 'env OBJECT_MODE=any llvm-nm'))
+
+# It is not realistically possible to account for all options that could
+# possibly be present in system and user configuration files, so disable
+# default configs for the test runs.
+config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"