The spill happened in the area of Cascade Drive and Molino Avenue, public works officials said.
The Southern Marin Fire Department was notified of the spill at 10:37 a.m., Battalion Chief Matt Barnes said. It was contained but not before some of the sewage leaked into the creek, Barnes said.
Samples of the water in the creek are being taken every two hours, and signs have been posted advising people to stay out of the water, Barnes said.
Public works officials said the spill was mitigated quickly.