Matthew McConaughey's just a beach bum these days ... who happened to find his way on stage with Jimmy Buffett to dance along to one of his legendary island jams.
The Oscar-winning actor's in Miami shooting for his upcoming film, "The Beach Bum" ... and showed off his groovy moves while Buffett played his hit song, "Volcano," during one of the scenes.
McConaughey plays a rebellious stoner named Moondog in the Harmony Korine film, and judging by the vid ... Moondog's in paradise when Buffett's rocking out.
"The Beach Bum" comes out in 2018.