The shooting was reported at 8:24 a.m. near Third Street and Palou Avenue.
Officers responded to multiple reports of shots fired and found the male victim, police spokesman Officer Albie Esparza said. He was taken to a hospital but Esparza did not know the extent of his injuries.
Esparza said a suspect has been arrested and a weapon was recovered near the shooting.
San Francisco Municipal Railway service has been disrupted in the area, he said.