A.J. McLean got to relive his Backstreet Boys glory days in front of a hard-partying crowd ... all thanks to a '90s cover band.
McLean hit up a '90s Throwback Party at Greystone Manor in West Hollywood Thursday night and when a '90s cover band started playing the BSB hit "Everybody," McLean went nuts.
Check out the video ... A.J. kills it on air guitar.
Backstreet's back ... sorta.