Gallagher died while under hospice care in the Palm Springs area early Friday morning, TMZ has learned. His manager tells TMZ Gallagher died of massive organ failure. He had been in ill health for a while, suffering numerous heart attacks over the years.

The rep tells TMZ, "Gallagher stayed on the road touring America for decades. He was pretty sure he held a record for the most stand-up dates, by attrition alone," adding, "While Gallagher had his detractors, he was an undeniable talent and an American success story."

Gallagher started working as a comedian after college ... getting his big break after appearing on Johnny Carson's 'Tonight Show' back in 1975 ... showing off his prop humor and quickly becoming one of the most popular and recognizable comedians in America.

His trademark bit was the "Sledge-O-Matic," where he'd use a large mallet to destroy a bunch of foods and objects -- always ending with a watermelon. While it started as a parody of the Veg-O-Matic commercials, it would go on to be what millions would remember him for.

Gallagher's since become a pop culture phenomenon, getting parodied and referenced over the years for his act ... most recently being played by fellow comedian Paul F. Tompkins in "Weird: The Al Yankovic Story."

The comedian had 17 specials spanning decades of his work ... with his first special, "An Uncensored Evening," debuting in 1980. He appeared in episodes of "Hollywood Squares," "WTF with Marc Maron," "Tosh.0" and "Celebrity Big Brother."

He had a health scare back in 2011, as we reported, he collapsed onstage during a performance in Minnesota. He also suffered a heart attack the following year in Texas, moments before he was supposed to perform.

We last saw the comedian back in 2014 ... and he offered us some fantastic advice for those looking to get someone a gift while stuck on a budget -- and it's as hilarious as you'd expect.

Gallagher, who was retired, is survived by his daughter, Aimee, and his son, Barnaby.

He was 76.