Former *NSYNC member Joey Fatone performed a heavy metal rendition of a classic '90s boy band song last night -- but it wasn't "Bye Bye Bye" or "Tearin' Up My Heart."

Joey was brought on stage at the Steel Panther show in L.A. -- where the band surprised him by playing Backstreet Boys' mega-smash "I Want it That Way."

Fatone went with it -- and totally rocked the song.

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