The former 'Charmed' actress saw the pedestrian punched by an "evil guy" in the Venice Beach area of the city, and rushed to aid him then called an ambulance.
She wrote on Twitter: "I just witnessed a 65 year old man get punched by an evil guy playing that goddamn knockout game. #criminal #heartbroken #whattheF?! Evil guy was wearing a mask. (sic)"
Rose, 40, added she had started out after the assailant but couldn't catch him, adding: "He was on a skateboard. Too fast. I chased."
Describing the state of the gentleman who had been attacked, she wrote: "Bleeding from the ear. Called an ambulance for him. His little dog tried to protect. :( (sic)"
A spate of so-called 'Knockout Game' incidents have occurred in the US recently, where young people pick on an unsuspecting older person at random and try to knock them out with a single punch, usually while filming it on a mobile phone.
Authorities have warned against such behavior and vowed to hand out heavy punishments to anyone caught attempting such a move.
Los Angeles Police Department's Pacific Division has been in touch with Rose to get more details about the incident, but the man responsible has yet to be named or located.