The theft occurred at about 2:40 p.m. on Sunday at Kaiser Permanente-South San Francisco, located at 1200 El Camino Real, according to police.
The woman was sitting alone in a main lobby when a man grabbed the phone out of her hands and ran, police said.
The suspect was described as standing around 5 feet 8 inches tall with a medium build. He was wearing a gray or blue hooded sweatshirt, light-colored blue jeans and black shoes at the time of the theft, police said.
He was last seen running north through the hospital parking lot next to El Camino Real.
Anyone with information about the crime is asked to contact South San Francisco police at (650) 877-8900.