A possible shooting was reported at 12:47 p.m. in Building 1 at the hospital located at 1001 Potrero Ave., Officer Albie Esparza said.
A person in the building reported hearing one shot fired and seeing a suspect flee from the building, Esparza said.
Police arrived with an "immense" response and evacuated Building 1 as well as nearby buildings 89 and 90 and instructed the rest of the campus to shelter in place, Esparza said.
Officers searched the three buildings but could find "no physical evidence of shots fired," Esparza said.
He said only one person heard the shot and that person was the only one who saw a suspect.
An all-clear was given at 1:50 p.m. and people were allowed to return to the evacuated buildings, Esparza said.
Building 1 primarily houses University of California at San Francisco research offices, hospital spokesman Tristan Cook said.
UCSF officials said on Twitter that the suspect was seen running across 22nd Street. Esparza said the possible suspect was described as a black man who is 5 feet 7 inches tall and wore jeans and a black hooded sweatshirt.