Brian Dennehy -- the veteran actor whose career spanned 5 decades on the stage and screen -- has died ... TMZ has learned.

Law enforcement sources tell us Dennehy died Wednesday night in Connecticut ... we're told he passed of natural causes and there will not be an official autopsy.

The actor's career began in the '70s with a run of small guest roles on TV shows like "Kojak," "M*A*S*H," "Lou Grant" and "Dallas" ... before landing a recurring role on "Dynasty" in 1981.

That led to his big breakout movie role the following year -- playing Sheriff Teasle in Sylvester Stallone's Rambo action flick, "First Blood."

He'd go on to star in several films in the '80s and '90s, including "Silverado," "Cocoon," "The Belly of an Architect," "Best Seller," "Presumed Innocent" and "Romeo + Juliet."

Brian's most notable comedy role came in 1995, when he played Big Tom Callahan in the classic Chris Farley-led laugh-fest, "Tommy Boy."

In more recent years, he voiced the character Django in "Ratatouille" and appeared on the hit show, "The Blacklist." All in all, he has nearly 200 film and television credits to his name ... and he won a Golden Globe for Best Performance by an Actor in 2001 for the TV movie, "Death of a Salesman."

Dennehy was also an accomplished theater actor, winning 2 Tony Awards for Best Actor ... his first for "Death of a Salesman" a couple years before his TV role, and the second for "Long Day's Journey Into Night" in 2003.

Play video content 11/13/13 TMZ.com

In fact, the last time we saw him in 2013, he was in L.A. doing a play ... and he had some good jokes.

Brian's survived by his wife and 5 children. He was 81.

RIP