diff --git a/synthtool/gcp/common.py b/synthtool/gcp/common.py
index ff102a484..4f4532031 100644
--- a/synthtool/gcp/common.py
+++ b/synthtool/gcp/common.py
@@ -369,6 +369,17 @@ def _load_repo_metadata(metadata_file: str = "./.repo-metadata.json") -> Dict:
 
 
 def _get_default_branch_name(repository_name: str) -> str:
+    """Read the default branch name from the environment.
+
+    First checks environment variable DEFAULT_BRANCH_PATH.  If found, it
+    reads the contents of the file at DEFAULT_BRANCH_PATH and returns it.
+
+    Then checks environment varabile DEFAULT_BRANCH, and returns it if found.
+    """
+    default_branch_path = os.getenv("DEFAULT_BRANCH_PATH")
+    if default_branch_path:
+        return Path(default_branch_path).read_text().strip()
+
     # This default should be switched to "main" once we've migrated
     # the majority of our repositories:
     return os.getenv("DEFAULT_BRANCH", "master")
diff --git a/tests/test_common.py b/tests/test_common.py
index 0e75185ca..446ec7c30 100644
--- a/tests/test_common.py
+++ b/tests/test_common.py
@@ -12,13 +12,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from synthtool.gcp.common import decamelize, _get_default_branch_name
+import os
+import tempfile
 from pathlib import Path
+from unittest import mock
+
 from pytest import raises
-import os
+
 import synthtool as s
+from synthtool.gcp.common import _get_default_branch_name, decamelize
+
 from . import util
-from unittest import mock
 
 MOCK = Path(__file__).parent / "generationmock"
 template_dir = Path(__file__).parent.parent / "synthtool/gcp/templates"
@@ -42,8 +46,17 @@ def test_handles_empty_string():
 
 
 def test_get_default_branch():
-    with mock.patch.dict(os.environ, {"DEFAULT_BRANCH": "main"}):
-        assert _get_default_branch_name("repo_name") == "main"
+    with mock.patch.dict(os.environ, {"DEFAULT_BRANCH": "chickens"}):
+        assert _get_default_branch_name("repo_name") == "chickens"
+
+
+def test_get_default_branch_path():
+    f = tempfile.NamedTemporaryFile("wt", delete=False)
+    fname = f.name
+    f.write("ducks\n")
+    f.close()
+    with mock.patch.dict(os.environ, {"DEFAULT_BRANCH_PATH": fname}):
+        assert _get_default_branch_name("repo_name") == "ducks"
 
 
 def test_py_samples_clientlib():