The actor took the stage at the Glastonbury Music Festival in England Saturday night during the band's headlining set ... and, ya gotta watch the video 'cause it's a tear-jerking tribute to the film and TV legend.

The vid kicks off with Chris and the crew jumping into their hit song "Fix You" -- Michael's already onstage, sitting in a wheelchair and holding a guitar on his lap.

After a couple minutes, CM heads over to MJF and sings part of the song from his knees so the two are at the same eye level ... you can see Michael's hands moving as he fingers and strums the instrument.

It's during an interlude in the band's hit song where Martin really highlights Fox though ... singing out the lyrics to Chuck Berry's "Johnny B. Goode" -- the song sci-fi fans know Fox's Marty McFly played during the climax of 'BTTF.'

Michael really comes alive at this part ... rocking out from his chair and even throwing a kick out just like he did way back in the 1985 film.

The real kicker ... Martin credits "Back to the Future" for encouraging Coldplay to start their band -- at which point the crowd breaks into raucous applause.

As you know ... Fox was diagnosed with Parkinson's way back in the early '90s -- and, he's mainly turned away from acting and toward activism in recent years as the disease has progressed. He gave fans an inside look at his life in his Emmy Award-winning 2023 documentary 'Still.'

That said, he's still made time to play with Coldplay over the years ... and, he's clearly still got some music skills in his repertoire.