Police and firefighters responded to a blaze on the east end of Hoover Park, located at 2901 Cowper St. at about 11 p.m.
A children's jungle gym was fully engulfed in flames, Detective Brian Philip said.
Firefighters extinguished the fire as quickly as they could, but the play structure was destroyed.
Residents in the area told police they saw juveniles in the park just before the fire. Investigators are working to determine whether the fire was arson, Philip said.
"It's odd that a play structure burned down but there's nothing to say it was intentional, although it's a great possibility," Philip said.
Anyone with information about the fire is asked to call Palo Alto Communications at (650) 329-2413 or e-mail anonymous tips to firstname.lastname@example.org.