1/20 10:23 AM PT -- Rick Harrison posted a tribute to Adam on his Instagram saying "You will always be in my heart! I love you Adam. 💔"

Fans and celebs left supportive comments on his post, including Donald Trump Jr., who stated "I'm so sorry man."

And, his brother Corey also took to Insta to post a sweet message with a pic of the two in the tub together as kids.

6:46 PM PT -- The Harrison family tells TMZ ... "Our family is extremely saddened by the death of Adam. We ask for privacy as we grieve his loss."

Rick Harrison's son Adam has died under tragic circumstances ... TMZ has learned.

A rep for the 'Pawn Stars' reality star tells us Adam recently died from a fatal overdose ... and we're told the family only just found out today. At this time, it's unclear where Adam was exactly when he passed -- and the specific circumstances are also unknown at this time.

Las Vegas Metro PD tells us they're investigating the case.

Adam is one of Rick's three kids -- all boys -- and he's been mostly off the radar in terms of Rick's show and store. Word is ... Adam helped out at the World Famous Gold & Silver Pawn Shop once upon a time -- but has since stepped away to do his own thing.

He doesn't appear to have been featured in any of the episodes from what we can see.

Rick had two kids with his first wife Kim ... Corey and Adam -- and he also had a son, Jake, with his second wife, Tracy.

While Rick's oldest son Corey is heavily involved in "Pawn Stars," first appearing on the reality show way back in 2009, Adam mostly remained out of the spotlight.

With Adam never really getting involved on the show, there's not a lot known about him ... though some folks have claimed he made a living as a plumber. We've heard he worked professionally in some kind of trade, but no word on what kind of job specifically.

Adam was only 39.

RIP