According to Sgt. Lyn Tomioka, the victim was stabbed near the intersection of Arguello Street and Geary Boulevard at about 2:45 p.m.
Witnesses were able to detain a suspect until police arrived and made an arrest, Tomioka said.
No information was immediately available about the suspect, or the circumstances surrounding the stabbing.
The victim's identity has not yet been determined, the medical examiner's office said this morning.
The stabbing is the first homicide reported by San Francisco police this year.
