The stabbing was reported at 3:33 p.m. near Pier 2, just south of the Ferry Building.
The victims, ages 21 and 22, got into a fight with two men in their 30s. The 21-year-old victim was stabbed in the torso, and the other victim was stabbed in the arm, according to police.
The victims waved down a taxi afterward and were taken to San Francisco General Hospital to be treated for their injuries.
The 21-year-old's wounds are considered life-threatening, police said. The other victim is expected to survive.
The suspects had not been found as of this morning.
Anyone with information about the stabbings is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.