diff --git a/pw_env_setup/bazel_only.json b/pw_env_setup/bazel_only.json
index bb6900a..3e5fa96 100644
--- a/pw_env_setup/bazel_only.json
+++ b/pw_env_setup/bazel_only.json
@@ -1,4 +1,5 @@
 {
+  "root_variable": "PW_ROOT",
   "cipd_package_files": [
     "pw_env_setup/py/pw_env_setup/cipd_setup/bazel.json",
     "pw_env_setup/py/pw_env_setup/cipd_setup/python.json",
diff --git a/pw_env_setup/compatibility.json b/pw_env_setup/compatibility.json
index d580972..1a02c07 100644
--- a/pw_env_setup/compatibility.json
+++ b/pw_env_setup/compatibility.json
@@ -1,4 +1,5 @@
 {
+  "root_variable": "PW_ROOT",
   "cipd_package_files": [
     "pw_env_setup/py/pw_env_setup/cipd_setup/pigweed.json",
     "pw_env_setup/py/pw_env_setup/cipd_setup/bazel.json",
diff --git a/pw_env_setup/config.json b/pw_env_setup/config.json
index 4101841..6702b31 100644
--- a/pw_env_setup/config.json
+++ b/pw_env_setup/config.json
@@ -1,4 +1,5 @@
 {
+  "root_variable": "PW_ROOT",
   "cipd_package_files": [
     "pw_env_setup/py/pw_env_setup/cipd_setup/pigweed.json",
     "pw_env_setup/py/pw_env_setup/cipd_setup/python.json",
diff --git a/pw_env_setup/docs.rst b/pw_env_setup/docs.rst
index e999b6c..eed3bc5 100644
--- a/pw_env_setup/docs.rst
+++ b/pw_env_setup/docs.rst
@@ -198,6 +198,11 @@
 
 .. _sample project: https://pigweed.googlesource.com/pigweed/sample_project/+/HEAD
 
+``root_variable``
+  Variable used to point to the root of the source tree. Optional, can always
+  use ``PW_PROJECT_ROOT`` instead. (That variable will be set regardless of
+  whether this is provided.)
+
 ``cipd_package_files``
   CIPD package file. JSON file consisting of a list of dictionaries with "path",
   "platforms", and "tags" keys. An example is below.
@@ -240,6 +245,7 @@
 .. code-block:: json
 
   {
+    "root_variable": "EXAMPLE_ROOT",
     "cipd_package_files": [
       "pigweed/pw_env_setup/py/pw_env_setup/cipd_setup/pigweed.json",
       "pigweed/pw_env_setup/py/pw_env_setup/cipd_setup/luci.json"
diff --git a/pw_env_setup/py/pw_env_setup/env_setup.py b/pw_env_setup/py/pw_env_setup/env_setup.py
index 02e3ffa..adf9cde 100755
--- a/pw_env_setup/py/pw_env_setup/env_setup.py
+++ b/pw_env_setup/py/pw_env_setup/env_setup.py
@@ -199,6 +199,7 @@
         self._virtualenv_gn_targets = []
         self._optional_submodules = []
         self._virtualenv_system_packages = False
+        self._root_variable = None
 
         self._config_file_name = getattr(config_file, 'name', 'config file')
         if config_file:
@@ -213,6 +214,8 @@
         self._use_existing_cipd = use_existing_cipd
         self._virtualenv_gn_out_dir = virtualenv_gn_out_dir
 
+        if self._root_variable:
+            self._env.set(self._root_variable, project_root)
         self._env.set('PW_PROJECT_ROOT', project_root)
         self._env.set('PW_ROOT', pw_root)
         self._env.set('_PW_ACTUAL_ENVIRONMENT_ROOT', install_dir)
@@ -248,6 +251,8 @@
     def _parse_config_file(self, config_file):
         config = json.load(config_file)
 
+        self._root_variable = config.pop('root_variable', None)
+
         self._optional_submodules.extend(config.pop('optional_submodules', ()))
 
         self._cipd_package_file.extend(