"Grease" star Jeff Conaway has died today ... several weeks after he was found unconscious in his L.A. area home ... TMZ has confirmed with the actor's family.

Conaway -- a notorious substance abuser -- had reportedly fallen into a coma after suffering an overdose earlier this month ... and doctors were not able to revive him.

Jeff's inner struggles became public when he appeared on "Celebrity Rehab with Dr. Drew" in 2008 and admitted to being addicted to cocaine, alcohol and painkillers.