The 19-year-old was born in Brussels but qualified to represent a host of countries, including Albania, Serbia and Turkey.
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"I have received official confirmation that Adnan Januzaj makes himself available for the Belgian national team for the rest of his career," he 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 World Cup squad this summer.