Hidden Valley Park was closed off for more than two hours Saturday after Martinez police discovered a suspicious device there.
Police were alerted to the device about 11:30 a.m. It was located in a grassy area near picnic tables in the middle of the park.
Sgt. Bryan Dodd said the device had the appearance of a pipe bomb.
Officers cordoned the area off while the Walnut Creek police bomb squad was called in.
The device was eventually determined not to be explosive and not to be a danger to the public.
Police reopened that portion of the park at about 2 p.m., Dodd said.