The assault inside the store was caught on surveillance camera.
One minute a customer was browsing through the electronics store. The next he was on the ground, decked by a man who swiped his iPhone and headed out the emergency exit.
Before the robbery, store security was tracking the man as he walked through the store, appearing more interested in other customers than the electronics products.
He then sneaks up on a man who is talking on his phone, punches him and knocks him to the ground and grabs his phone.
Los Angeles County Sheriff's Department Sgt. Jeff Bishop said while the victim was rattled, he is fortunate he wasn't more seriously injured.
"People get hit, unsuspecting like that, they fall down, you could be critically injured, paralyzed or even killed if you land or fall the wrong way," Bishop said.
Authorities have released video and stills from the attack and are hoping someone will recognize the suspect.