EXCLUSIVE

Sib Hashian -- the drummer for the rock group Boston -- died after collapsing onstage Wednesday night ... TMZ has learned.

Sib's son, Adam, tells us the drummer was on the Legends of Rock Cruise and was midset when he suddenly collapsed. A witness says CPR was performed and a defibrillator was used but to no avail.

The L.O.R. Cruise began on March 18 in Florida and had stops in several places including Puerto Rico and the Bahamas. We're told the cruise will continue and other musicians are performing tributes to Sib on the boat Thursday.

The drummer -- who donned an EPIC afro in the early days of his stellar career -- was famously part of Boston's self-titled album, which featured the mega-hit "More Than a Feeling."

Sib's daughter, Lauren, has a daughter with Dwayne Johnson.

He was 67.