Police are worried a second child in the car may still be at risk.
The couple, who may be the parents, left the child at the rest stop at Great Highway and Sloat Blvd.
A source with knowledge of the investigation says the child is about 5 years old. The same source says the couple is from Arkansas and that there be another child with them.
The abandonment was on a lot of people's minds at the beach on Tuesday.
Shawna Watson came here from Turlock with her 18-month-old daughter, Ava. "They have another child and that child saw you dump that baby? That says what if I'm next? That makes my blood boil. Nobody should abandon a poor child," she said.
John Stanley and his son, Jack, were also at the beach. He was shocked. "I can't imagine how a parent can do that to their kid. Crazy," he said.
Chandra Johnson is with the Human Services Agency. She wants the public to know there are places couples can take their children if they need help.
"It's really using better practices of hospitals, fire stations, schools. Somewhere you can speak to someone who understands the network of children's protective services. They can make sure the child is in immediate care and not immediate danger out in public," Johnson said.
Police say there are no physical injuries to the child, who is with Children's Protective Services.
Anyone with information can call the department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411 with "SFPD" at the start of the message.
A young child about 5 years old abandoned on Sloat and Great Highway in SF last nite by an Arkansas couple. Police looking for the parents. pic.twitter.com/lrIgZAL1Ey— Vic Lee (@vicleeabc7) June 6, 2017