The victim was walking to her car outside a market in the 100 block of El Camino Real at about 6:10 p.m. when she was approached by man wearing a blue bandana over his face, according to South San Francisco police.
The suspect demanded money and put his right hand inside his pocket, simulating a weapon, police said.
The victim believed the man was armed and gave him an undisclosed amount of cash, police said.
She was not injured.
The suspect, who appeared to be in his late teens to early 20s, was last seen walking toward Huntington Avenue.
He was wearing a black hoodie and black jeans.
Anyone with information about the robbery is asked to contact South San Francisco police at (650) 877-8900.