Raymon Abad, 27, surrendered on Friday, the day after San Francisco police released a photo of him and another suspect in the beating, which occurred on Jan. 29 near Polk and Elm streets, police spokesman Officer Albie Esparza said.
Police said that at about 2 a.m. that day, the victim, a 23-year-old Hayward man, saw some men harassing women in the street and tried to intervene.
The men kicked the victim and knocked him to the ground, then continued to punch and kick him until he lost consciousness, according to police.
The victim was critically injured and was taken to San Francisco General Hospital. He has since been released from the hospital but remains in serious condition and may not fully recover from his injuries, police said.
The suspects fled, but investigators tracked down a photo of the pair believed to be responsible for the beating. The picture also included a person of interest in the case.
Abad turned himself in on Friday, and was arrested on suspicion of felony assault and battery, Esparza said.
The second suspect is described as an Asian man about 5 feet 6 inches tall who weighs about 135 pounds. He remains at large.
Anyone with information about the suspect's identity is urged to call Inspector James Garrity of the Police Department's criminal investigations unit at (415) 553-9002.
People wishing to remain anonymous can call the department's tip line at (415) 575-4444 or send a tip by text message to TIP411.