The 19-year-old was born in Brussels but qualified to represent a host of countries including Albania, Serbia and Turkey.
Januzaj's international career had been subject to much speculation after he broke into the first-team set-up at Old Trafford this season, but Wilmots has claimed the teenager has chosen to play for Belgium.
"I have received official confirmation that Adnan Januzaj makes himself available for the Belgian national team for the rest of his career," Wilmots posted on his official Twitter account.
"This is good news and I am delighted that such a talented player can be selected for (Belgium)"
Januzaj has become a regular at United during what has been a tumultuous season for the reigning Premier League champions - culminating in the sacking of manager David Moyes on Tuesday.
He has made 32 appearances for the club in all competitions, scoring four times, and could now be in line for a place in Belgium's FIFA World Cup squad.
BACK OF THE NETAdnan Januzaj to represent Brazil in 1982
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.