Smash Mouth's Steve Harwell on Deathbed, Only Days to Live

Steve Harwell is experiencing the last chapter of his life -- this after battling medical complications for years -- and his family is saying their goodbyes ... TMZ has learned.

A manager for the Smash Mouth lead singer tells us Steve has been struggling with alcohol abuse throughout his life -- and now, he's reached the final stage of liver failure ... for which he'd, up until recently, been receiving treatment at a hospital.

At this point, we're told SH is in hospice at home ... where his friends and loved ones have gathered over the past 3 days. Steve's manager says he likely has only a week or so to live.

This news comes almost 2 years after a troubling incident involving Steve ... where he appeared incredibly disoriented during a live performance, slurring his words and yelling at the audience. It was concerning at the time, and shortly thereafter ... he said he was retiring.

At the time, sources close to Steve told us he'd been battling ongoing health issues -- stemming from a number of diagnoses he'd received over the years, including cardiomyopathy, heart failure and Wernicke's encephalopathy. His substance abuse was also mentioned to us back then ... which we were told had contributed to his declining state.

It's an incredibly sad development for fans of his ... and for music lovers everywhere.

Smash Mouth contributed some of the biggest pop culture hits in the early 2000s -- including, of course, "All Star," "I'm a Believer" and others ... some of which were featured in "Shrek."