Metallica frontman James Hetfield is a huge Raiders fan -- and says he's "a little pissed" that his team is ditching Oakland for Sin City.

FYI, Hetfield actively campaigned for the Raiders to stay in the Bay Area -- and publicly supported Ronnie Lott in his efforts to build a new stadium for the team in Oakland.  

But those plans didn't work out and Raiders owner, Mark Davis, ultimately got league approval to move the team to Las Vegas. The team is expected to relocate in 2019.

Hetfield says he'll continue to support the Silver and Black (he stuck by them during the L.A. years) ... but it's clear he's still bummed.