Authorities said the victim was stabbed several times on his upper body at about 10 p.m. in the parking lot of the 24 Hour Fitness at the Buena Park Downtown Mall. He was transported to UC Irvine Medical Center in critical condition, where he died Wednesday morning.
The victim was identified as 22-year-old Brian Lee of Buena Park.
There was no word on a motive, and police say the attack was not believed to be gang-related.
Officials said the suspects were four men dressed in dark-colored clothing. They were seen leaving the southeast corner of the mall in a dark-colored four-door sedan.
Investigators are reviewing security camera video, hoping to identify suspects.
"Nobody could really give us any definitive descriptions of the perpetrators. All we have to go on is a dark, four-door sedan vehicle," said Buena Park Police Lt. Gary Worrall.
Several witnesses heard screams and helped the victim.
"When I got there, he was already on the ground. We started asking his name, questions, to see if he had consciousness," said witness Justin Chavers.
Investigators said it's unusual for this type of crime to occur in the area.
Anyone with information related to the case was urged to call Buena Park Police at (714) 562-3902.