The accident was reported at 4:48 p.m. at Clement Street and 18th Avenue.
The 39-year-old bicyclist was traveling east on Clement Street when she collided with the USPS truck, which was headed north on 18th Avenue, police spokesman Officer Albie Esparza said.
The truck apparently did not come to a complete stop at the intersection, Esparza said.
The bicyclist, who was not wearing a helmet, was taken to San Francisco General Hospital to be treated for head injuries. She was released later in the day, Esparza said.
The truck driver was not cited, and the accident remains under investigation, according to Esparza.