Robert Downey Jr.'s son was arrested for cocaine possession in West Hollywood Sunday afternoon ... law enforcement sources tell TMZ.
We're told Indio Downey was a passenger in a car at around 2 PM PT when cops drove up along side the vehicle and noticed the passenger was smoking something out of a pipe.
An L.A. County Sheriff's deputy pulled the car over, did a search and allegedly found cocaine in Indio's possession. He was arrested and taken to jail for possession of cocaine.
20-year-old Indio -- who appeared in "Kiss Kiss Bang Bang -- posted his $10K bail and was released just after midnight Sunday.
Robert Downey Jr. famously struggled with drug addiction and ended up in prison. The drugs that got him in so much trouble -- cocaine and heroin.