Credit: Jeffrey Mayer/

Backstreet's rehab.

Backstreet Boys member A.J. McLean has checked into rehab for the third time, a source confirms to (TMZ was first to report the news.)

McLean, 33, and his bandmates have been prepping to hit the road with New Kids on the Block.

"He wanted to go in before they opened their new tour," the source explains.

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Reps for the singer -- he spent time in rehab in 2001 and 2002 for alcohol and cocaine abuse -- had no comment.

McLean Tweeted over the weekend:  "I love u all and wish me luck! I start my sober journey [tomorrow]...See u on the other side!"