The rock band's longtime frontman -- who was one of the original founding members dating back to 1994 -- passed away Monday morning at his home in Boise, Idaho as a result of liver failure -- this according to his manager.

We're told he was surrounded by family and friends and passed peacefully and comfortably.

TMZ broke the story ... Steve had been in hospice and was given only days to live this past weekend. As such, he was receiving end-of-life care at home with friends/family. We'd been told he succumbed to his health battles over alcohol abuse, among other ailments.

Remember, SH announced he was retiring from performing a couple years ago following a show in which he was found to have been slurring his words and acting erratic onstage.

His passing marks a tragic day for Smash Mouth fans -- of which there are many. His band skyrocketed to fame in the late '90s and early 2000s, on account of their album "Astro Lounge" ... which featured some of their most famous songs, like "All Star."

Of course, their involvement in the "Shrek" film put them on another level of fame -- as did their cover of The Monkees' hit song "I'm a Believer." Around this time, they were huge. Other notable tracks that Steve contributed to ... "Walkin' On the Sun," "Then the Morning Comes," "Can't Get Enough of You Baby," "Pacific Coast Party" and "Why Can't We Be Friends."

Steve dealt with hardships through much of his life -- his infant son died at only 6 months due to leukemia -- and he himself had been diagnosed with illnesses over the years too.

Despite this, he carried on with a smile and did what he loved in music and performing ... bringing joy to many, many listeners throughout his career. Steve was only 56.