"For personal reasons, I've checked myself into a rehab clinic," McLean says in a statement. "With 2011 being a busy year for Backstreet, I want to be healthy and perform and sing for people at my very best and happiest. None of this will affect our current tour plans and I look forward to seeing everyone really soon! Thank you for all of your support and respecting my family and friends privacy during this time."
McLean, 33, had previously offered clues to his intentions, Tweeting Sunday that "Its gonna be a great year! I love u all and wish me luck! I start my sober journey tomm!"
And then on Monday, he bid his followers farewell: "See u on the other side!"
While he gave no reason why he is seeking treatment, the singer has been in rehab before in 2001 and, again, in 2002 for issues including depression and alcohol abuse.
In 2001, he sought treatment for depression, anxiety and excessive consumption of alcohol. At the time, his bandmate Brian Littrell said McClean was well aware of his issues, which included staying out late partying and then missing work commitments the next day.
"He came to us [and said], 'Guys, I have a problem, and I need to get help,' " Littrell said at the time.
McLean, who took a turn as a solo artist in 2008, is gearing up for a busy summer, during which the Backstreet Boys and New Kids on the Block are due to go on tour together.
TMZ.com was the first to report the rehab news.