Robert Cosby Jr. — the son of “The Real Housewives of Salt Lake City” star Mary Cosby — is dead … TMZ has learned.

Robert died Monday in Utah … the cause of death is unclear, but police responded to a possible overdose, and it turned into a death investigation upon arrival.

Police in Salt Lake City tell us they responded Monday night to a call for a “full arrest/medical emergency” for a 23-year-old male. We’re told fire and paramedics were dispatched.

His death comes only a couple months after Robert’s wife, Alexiana Smokoff, filed for divorce.

Robert appeared on ‘RHOSLC’ a few times over the years … during Season 4, there was a storyline about Mary learning Robert had secretly gotten married right under her nose. The wedding reportedly went down in August 2022 at a Utah courthouse.

‘RHOSLC’ made Robert and Alexiana’s alleged drug problems a big part of Mary’s storyline, too.

Robert had numerous legal issues … as we first reported, he was arrested in Utah back in September for a number of alleged crimes, including trespassing and assault. He was previously arrested for DUI in 2022, when cops said he didn’t have a license and disobeyed traffic laws. He ended up pleading guilty to the DUI.

He was only 23.