Leonardo Guerrero was found by a Sonoma County sheriff's deputy who was riding an off-road motorcycle in the adjacent Spring Lake Park around 8:30 a.m., Sgt. Clay Van Artsdalen said.
The boy was cold and tired but in good health and spirits, and was medically cleared to return to his family, Van Artsdalen said.
Guerrero had gone to Howarth Park in east Santa Rosa with his mother around 5 p.m. Monday, Sgt. Michael Clark said.
She had lost sight of him as he was riding his bicycle on one of the trails that led from the main parking lot, Clark said.
The boy's mother searched for her son on foot, then returned to the parking lot and called police around 8:30 p.m., Clark said.
Police learned that Guerrero had depression issues and the cognitive abilities of a 10-year-old, Clark said.
The California Highway Patrol's helicopter searched the park trails and the Sonoma County Search and Rescue Team began its search at midnight, Clark said.
The 5,000-acre park has over 40 miles of trails.
Search-and-rescue assistance also was provided by Contra Costa, Marin and San Mateo counties, Clark said.