FRESH PRINCE' DAD JAMES AVERY DIES DEAD AT 68

James Avery -- the beloved dad on "The Fresh Prince of Bel-Air" -- has died at age 68 ... TMZ has learned.

Sources close to the actor tell TMZ he died in an Glendale, CA hospital last night. Avery had undergone open heart surgery and took a turn for the worse late yesterday.  

And we're told ... his wife, Barbara, had been at his bedside but left for a short period of time and when she came back ... she learned he had just died.

Alfonso Ribeiro -- who played Avery's son on "Fresh Prince" -- just posted a message on his Facebook page.