Evan Flanary was reported missing by family members who said he has not been heard from since leaving to meet a stranger who he contacted on a social media phone app at about 5:30 p.m., according to San Francisco police.
Flanary has blond hair and blue eyes, stands about 6 feet 1 inch tall and weighs about 160 pounds.
He was last seen wearing blue jeans, a gray hooded sweatshirt, a "Cal" baseball hat and a backpack.
Flanary has no history of medical or psychological conditions and was not on any form of medication, according to police.
Anyone who might have information regarding Flanary's whereabouts is asked to contact the San Francisco police missing persons unit at (415) 558-5508.