diff --git a/src/gn/setup.cc b/src/gn/setup.cc
index d1ba197..1535276 100644
--- a/src/gn/setup.cc
+++ b/src/gn/setup.cc
@@ -122,6 +122,9 @@
       cause the file //BUILD.gn to be loaded. Note that build_file_extension
       applies to the default case as well.
 
+      The command-line switch --root-target will override this value (see "gn
+      help --root-target").
+
   script_executable [optional]
       Path to specific Python executable or other interpreter to use in
       action targets and exec_script calls. By default GN searches the
@@ -840,17 +843,32 @@
   }
 
   // Root build file.
-  const Value* root_value = dotfile_scope_.GetValue("root", true);
-  if (root_value) {
-    if (!root_value->VerifyTypeIs(Value::STRING, err)) {
-      return false;
-    }
-
+  if (cmdline.HasSwitch(switches::kRootTarget)) {
+    auto switch_value =
+        cmdline.GetSwitchValueASCII(switches::kRootTarget);
+    Value root_value(nullptr, switch_value);
     root_target_label = Label::Resolve(current_dir, std::string_view(), Label(),
-                                       *root_value, err);
+                                       root_value, err);
     if (err->has_error()) {
       return false;
     }
+    if (dotfile_scope_.GetValue("root", true)) {
+      // The "kRootTarget" switch overwrites the "root" variable in ".gn".
+      dotfile_scope_.MarkUsed("root");
+    }
+  } else {
+    const Value* root_value = dotfile_scope_.GetValue("root", true);
+    if (root_value) {
+      if (!root_value->VerifyTypeIs(Value::STRING, err)) {
+        return false;
+      }
+
+      root_target_label = Label::Resolve(current_dir, std::string_view(),
+                                         Label(), *root_value, err);
+      if (err->has_error()) {
+        return false;
+      }
+    }
   }
   // Set the root build file here in order to take into account the values of
   // "build_file_extension" and "root".
diff --git a/src/gn/switches.cc b/src/gn/switches.cc
index 2b42e13..b94bc47 100644
--- a/src/gn/switches.cc
+++ b/src/gn/switches.cc
@@ -60,7 +60,7 @@
 const char kDotfile_Help[] =
     R"(--dotfile: Override the name of the ".gn" file.
 
-  Normally GN loads the ".gn"file  from the source root for some basic
+  Normally GN loads the ".gn" file from the source root for some basic
   configuration (see "gn help dotfile"). This flag allows you to
   use a different file.
 )";
@@ -175,6 +175,37 @@
   gn desc //out/Default --root="C:\Users\BObama\My Documents\foo"
 )";
 
+const char kRootTarget[] = "root-target";
+const char kRootTarget_HelpShort[] =
+    "--root-target: Override the root target.";
+const char kRootTarget_Help[] =
+    R"(--root-target: Override the root target.
+
+  The root target is the target initially loaded to begin population of the
+  build graph. It defaults to "//:" which normally causes the "//BUILD.gn" file
+  to be loaded. It can be specified in the .gn file via the "root" variable (see
+  "gn help dotfile").
+
+  If specified, the value of this switch will be take precedence over the value
+  in ".gn". The target name (after the colon) is ignored, only the directory
+  name is required. Relative paths will be resolved relative to the current "//"
+  directory.
+
+  Specifying a different initial BUILD.gn file does not change the meaning of
+  the source root (the "//" directory) which can be independently set via the
+  --root switch. It also does not prevent the build file located at "//BUILD.gn"
+  from being loaded if a target in the build references that directory.
+
+  One use-case of this feature is to load a different set of initial targets
+  from project that uses GN without modifying any files.
+
+Examples
+
+  gn gen //out/Default --root-target="//third_party/icu"
+
+  gn gen //out/Default --root-target="//third_party/grpc"
+)";
+
 const char kRuntimeDepsListFile[] = "runtime-deps-list-file";
 const char kRuntimeDepsListFile_HelpShort[] =
     "--runtime-deps-list-file: Save runtime dependencies for targets in file.";
@@ -298,6 +329,7 @@
     INSERT_VARIABLE(Markdown)
     INSERT_VARIABLE(NoColor)
     INSERT_VARIABLE(Root)
+    INSERT_VARIABLE(RootTarget)
     INSERT_VARIABLE(Quiet)
     INSERT_VARIABLE(RuntimeDepsListFile)
     INSERT_VARIABLE(ScriptExecutable)
diff --git a/src/gn/switches.h b/src/gn/switches.h
index 69c47b6..2f89e50 100644
--- a/src/gn/switches.h
+++ b/src/gn/switches.h
@@ -74,6 +74,10 @@
 extern const char kRoot_HelpShort[];
 extern const char kRoot_Help[];
 
+extern const char kRootTarget[];
+extern const char kRootTarget_HelpShort[];
+extern const char kRootTarget_Help[];
+
 extern const char kRuntimeDepsListFile[];
 extern const char kRuntimeDepsListFile_HelpShort[];
 extern const char kRuntimeDepsListFile_Help[];