At about 6:20 p.m., the man was seen on fire, running out of the Golden Gate Adult Superstore at 99 Sixth St., police Lt. Troy Dangerfield said.
Dangerfield did not know how the man caught fire.
The man collapsed at Mission and Sixth streets, where he was spotted by a private ambulance crew who came and extinguished the flames, fire Lt. Mindy Talmadge said.
The man was taken to St. Francis Memorial Hospital to be treated for his injuries, Dangerfield said.
Talmadge said the man suffered burns over 90 percent of his body, varying from first- to third-degree burns.