diff --git a/src/functions_framework/_cli.py b/src/functions_framework/_cli.py
index 48455ea6..aed69116 100644
--- a/src/functions_framework/_cli.py
+++ b/src/functions_framework/_cli.py
@@ -32,13 +32,26 @@
 @click.option("--host", envvar="HOST", type=click.STRING, default="0.0.0.0")
 @click.option("--port", envvar="PORT", type=click.INT, default=8080)
 @click.option("--debug", envvar="DEBUG", is_flag=True)
+@click.option(
+    "--env",
+    "env_vars",
+    multiple=True,
+    metavar="KEY=VALUE",
+    help="Set a runtime environment variable for local execution.",
+)
 @click.option(
     "--asgi",
     envvar="FUNCTION_USE_ASGI",
     is_flag=True,
     help="Use ASGI server for function execution",
 )
-def _cli(target, source, signature_type, host, port, debug, asgi):
+def _cli(target, source, signature_type, host, port, debug, env_vars, asgi):
+    for env_var in env_vars:
+        key, separator, value = env_var.partition("=")
+        if not key or not separator:
+            raise click.BadParameter("must be in KEY=VALUE format", param_hint="--env")
+        os.environ[key] = value
+
     if asgi:
         from functions_framework.aio import create_asgi_app
 
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 75c93f20..edc09462 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -130,6 +130,34 @@ def test_cli(monkeypatch, args, env, create_app_calls, run_calls):
     assert wsgi_server.run.calls == run_calls
 
 
+def test_cli_sets_runtime_env(monkeypatch):
+    wsgi_server = pretend.stub(run=pretend.call_recorder(lambda *a, **kw: None))
+    wsgi_app = pretend.stub(run=pretend.call_recorder(lambda *a, **kw: None))
+    create_app = pretend.call_recorder(lambda *a, **kw: wsgi_app)
+    monkeypatch.setattr(functions_framework._cli, "create_app", create_app)
+    create_server = pretend.call_recorder(lambda *a, **kw: wsgi_server)
+    monkeypatch.setattr(functions_framework._cli, "create_server", create_server)
+    monkeypatch.delenv("LOCAL_ONLY", raising=False)
+    monkeypatch.delenv("EMPTY_VALUE", raising=False)
+
+    runner = CliRunner()
+    result = runner.invoke(
+        _cli, ["--target", "foo", "--env", "LOCAL_ONLY=1", "--env", "EMPTY_VALUE="]
+    )
+
+    assert result.exit_code == 0
+    assert os.environ["LOCAL_ONLY"] == "1"
+    assert os.environ["EMPTY_VALUE"] == ""
+
+
+def test_cli_rejects_invalid_runtime_env():
+    runner = CliRunner()
+    result = runner.invoke(_cli, ["--target", "foo", "--env", "LOCAL_ONLY"])
+
+    assert result.exit_code == 2
+    assert "must be in KEY=VALUE format" in result.output
+
+
 def test_asgi_cli(monkeypatch):
     asgi_server = pretend.stub(run=pretend.call_recorder(lambda *a, **kw: None))
     asgi_app = pretend.stub()