Bobbi Kristina's BF Nick Gordon Died of Heroin Overdose

Nick Gordon died of a heroin overdose ... according to his autopsy and toxicology report.

The report -- obtained by TMZ -- says Gordon was found unresponsive and in cardiac arrest. He was hospitalized for 7 hours before he died. There was also Narcan in his system ... a drug used to counteract the effects of an overdose.

As we reported ... Nick had some sort of black fluid foaming out of his mouth the day someone called 911 to say he wasn't breathing. A man claiming to be Nick's brother said Nick died of an apparent OD. The "black stuff" could support that conclusion ... given that black discharge is sometimes related to a foam cone that comes out of the mouth and nostrils after an opioid OD.

The 911 call came in as "cardiac respiratory" on New Year's Day. He had been rushed to the hospital where he died. Nick's brother, Jack Walker Jr., shared the news by writing, "GOD WHY DID I HAVE TO LOSE MY BROTHER ON NEW YEARS," and adding, "All I can do is cry."

As you know by now ... Nick was present when Whitney Houston's daughter and his girlfriend -- Bobbi Kristina -- OD'd in a bathtub in 2015.