Jimmy Kimmel's Band Leader Cleto of Cleto and the Cletones Dead

Tragedy for friends and family ... Jimmy Kimmel just announced his show's longtime band leader and childhood friend, Cleto Escobedo III, died Tuesday morning.

Jimmy paid tribute to him in an IG post Tuesday ... saying he's heartbroken, especially since he and Cleto had been inseparable since Jimmy was 9 years old.

Jimmy writes ... "The fact that we got to work together every day is a dream neither of us could ever have imagined would come true. Cherish your friends and please keep Cleto’s wife, children and parents in your prayers."

The cause of Cleto's death was not released.

Escobedo had been noticeably MIA from the ABC show for months after falling ill. Reports state that Kimmel even canceled his Thursday episode of "Jimmy Kimmel Live!" last minute to be by Escobedo’s side.

Jimmy and Cleto grew up across the street from each other in Las Vegas. When Kimmel landed his show back in 2003, he fought hard to get Escobedo on board as the leader of the sextet -- even taking the ABC President to see him perform.

He landed the gig, and Kimmel later said, "I wanted somebody I had chemistry with. And there’s nobody in my life I have better chemistry with than him."

Cleto was 59.