Maintenance workers spotted the cat, which they estimated at about 90 to 100 pounds, in San Ramon Creek at 8 a.m., according to Danville police.
The animal did not display any aggressive behavior and moved away down the creek, police reported.
Responding officers were not able to locate the cat, but area schools were notified to limit student access to the park. In addition, the park will now be closed from 8 p.m. to 8 a.m. until further notice.
The California Department of Fish and Game is now monitoring the area in an attempt to locate the cat, and police are notifying area residents about the sighting.
Police are asking anyone who sees a mountain lion to call 911 or Danville police at (925) 820-2144.