The incident happened Wednesday night and was caught on surveillance camera.
"Pulls out a blunt metal object and, almost like a baseball swing, swings it across the left side of the victim’s head, knocking him unconscious," said San Francisco Police Officer Carlos Manfredi.
The victim, who lost a lot of blood and needed 50 stitches, is known to keep to himself.
"He's a very nice man. He doesn't try to harm anyone. He always says hi to everyone who says hi to him," Manfredi said.
His name is Robert, said the owner of a nearby bar who claims to know him well.
"The Robert victim, he's a regular guy," Jung Lee told ABC News. "He sit here morning, day, night. You know, every day, maybe a decade, 10 years."
Police are urging the public to look at the video and help identify the attacker.
"We want to send a strong message to anyone who thinks it's OK to do this. If we catch you, you will be incarcerated," Manfredi added.
The suspect could be charged with attempted murder.
