Authorities responded to a shopping center near the 100 block of Bradford Avenue about 8:30 a.m. Surveillance video shows the victim sleeping when someone comes up and stabs him, police said.
The victim, identified only as a 53-year-old man, was declared dead at the scene.
Investigators were unable to make out the suspect's face in surveillance video.
Neighbors said the man was a well-known transient who lived in the area for the past several years.
"I just saw him two days ago and he was fine and doesn't' bother anybody," said nearby resident Eileen Jackson. "Unfortunately, you've got people out here who think it's just funny and like to hurt these guys and it's not right."
Residents are just as confused as police as to why someone would kill the man.
"He was not a violent person, never bothered anyone, just a very humble man," said Edgar Pineda of Placentia.
Anyone with information is asked to call Placentia police at (714) 993-8164.