9:09 AM PT -- Sources close to Harrison tell us he passed after losing his battle with Parkinson's Disease.

Richard Harrison -- famously known as "Old Man" on "Pawn Stars" -- has died, according to his son, Rick Harrison.

'''The Old Man' Harrison passed away this morning surrounded by those he loved," Rick said Monday morning ... "He will be tremendously missed by our family, the team at Gold & Silver Pawn and his many fans the world over."

"He was my hero and I was fortunate to get a very cool ‘Old Man’ as my dad."

The circumstances surrounding his death are unclear. However, Harrison's appearances on the show over the last couple years were slim to none.

'Old Man' moved to Vegas back in the early '80s and opened the Gold & Silver Pawn Shop with his son, Rick.

When the producers found the family and turned their daily lives into a show, it became an instant success. It's been on the air since July 2009 with 'Old Man' as one of the fan favorites.

A rep for History Channel tells TMZ, "We are deeply saddened by the loss of our friend Richard ‘The Old Man’ Harrison, a beloved member of the HISTORY and ‘Pawn Stars’ family. He will be greatly missed for his wisdom and candor."

Harrison is a Navy veteran and often talked about his military career on the show.