The accident was reported at about 7:25 p.m. near the intersection of 19th Avenue and Junipero Serra Boulevard.
The 19-year-old pedestrian was struck by a Muni vehicle on the M-Ocean View line. She suffered head trauma and was taken to San Francisco General Hospital, according to police.
Police initially said the victim was in serious but stable condition, but police Sgt. Mike Andraychak said this afternoon that she is now in critical condition.
The driver of the Muni vehicle has been placed on non-driving status and will undergo drug and alcohol testing, per standard procedure, agency spokesman Paul Rose said.
The cause of the crash remains under investigation by police and Muni officials.