The actor's manager, Sandy Joseph, tells us Walsh passed away at a hospital in Vermont Tuesday as a result of cardiac arrest. No word on what exactly landed him in the hospital, or what sort of ailment he might've been battling.

Walsh was a veteran in Hollywood, and one of the most prolific character actors in the biz -- having racked up a whopping 233 acting credits to his name, with roles dating back to the '60s, playing Jason Randall on an episode of "The Doctors."

Since then, he'd starred in countless well-known films and TV shows spanning decades.

In addition to his work on 'CWTK' -- where he played Tim Allen's cranky neighbor and archenemy, Walt Scheel -- Walsh has acted in flicks like "Ordinary People," "The Jerk," "Fletch," "Blood Simple," "A Time to Kill," "Romeo + Juliet," "Wild Wild West," and many, many more.

His TV work is equally impressive ... Walsh had worked on TV series like "Starsky and Hutch," "Baretta," "The Rockford Files," "The Sandy Duncan Show," "East of Eden," "Little House on the Prairie," "The Twilight Zone," "Unsub," "The Flash," "Home Improvement," "The X-Files," "NYPD Blue," "Big Guy and Rusty the Boy Robot," "Damages," "Adventure Time" and others.

Walsh is survived by his nieces and nephews. He was 88.