Smash Mouth Singer Steve Harwell on Hiatus Due to Heart Issues

Fear not, Smash Mouth fans ... Steve Harwell's recent absence is due to health conditions, but he's on the mend and will soon hit the stage again.

A rep for the American rock band tells TMZ ... Steve had not been feeling well due to issues with cardiomyopathy and heart failure, which forced him to miss a few performances. Steve's had cardiomyopathy -- a form of heart disease where the heart muscle becomes enlarged and weak, making it difficult to pump blood -- since 2015.

Fans were speculating Smash Mouth fired Steve. Someone recently tweeted the band asking, "Hey @smashmouth did y'all replace your singer & not say anything?" Someone else wrote, "ok a couple things 1) @smashmouth did you really fire your singer and B) can I be your new singer."

The answer's an unequivocal "No" to all those questions. We're told Steve manages his heart condition with meds, but recently some issues came up and things needed to be adjusted. You'll recall he was rushed to the hospital in 2017 due to his heart condition.

The band's current singer is someone the band knows and was substituting for a few shows, but it's NOT a permanent move.