"When San Francisco police responded, they found him unconscious and he was taken to an area hospital with life-threatening injuries," said Officer Adam Lobsinger.
The security guard was assaulted while working outside the building along California Street. Police say skateboarders have been linked to the incident.
"There was a large group of skateboarders leaving the scene, but as of right now it's an early investigation," Lobsinger said.
The building makes it clear skateboarding isn't allowed. There are signs posted in several locations.
Richard Zhao and others told ABC7 News they often see skateboarders violating the signage.
"Sometimes they do violent dangerous moves, right? They can hurt people," Zhao said.
Others said they've seen guards shoo skaters away as they try to use the rails and stairs to skate.
Thibaut Fournie and his friend sitting near the stairs with their skateboards on Monday. They were unaware of Sunday's incident.
"I really feel sorry for the guard," Fournie said. "I hope he's going to be okay."
Fournie fears the assault could cause people to jump to conclusions about skateboarders.
"If it was the skateboarders, it doesn't mean all the skateboarders are like this," he said.
It is possible someone recorded the assault of the security guard. Police are asking anyone with information to speak up.