Meng Linggong, 69, of San Jose, was reported missing around 11:20 p.m. Monday, about four hours after his family began looking for him.
After police found Linggong's cellphone around 2:40 a.m. Tuesday in the park, search-and-rescue units responded to assist. A tracking dog is also scouring the area, according to police.
Linggong was last seen wearing dark pants and a dark shirt. He is 5 feet 7 inches tall with black hair, and weighs 111 pounds, according to police.
Police say they have already checked local hospitals for Linggong, as well as the jail and psychiatric facilities. Investigators do not suspect foul play, police said.
Police are asking anyone with information about Linggong's whereabouts to call 911 or (408) 277-8911, and to reference case No. 12-149-1012.