'RHOSLC' Star Mary Cosby's Son Robert Cosby Jr. Dead at 23

11:20 AM PT -- Mary Cosby and Robert Sr. tell TMZ … "Our beloved son Robert Jr. has been called home to the Lord. Though our hearts ache, we take comfort in God’s promise and in knowing he is finally at peace. We are grateful for your prayers and trust in the Lord to carry us through this time of sorrow."

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 got out of prison in February.

He was only 23.