The 18-year-old, who was born in Tampa, Florida, played for Germany in three qualifiers for the 2014 European Under-19 Championship, making his debut last Oct. 10 against Belarus.
He made his professional debut for Bayern on Nov. 27 as an 88th-minute substitute in a Champions League match at CSKA Moscow.
Since 2009, FIFA has allowed a one-time change in national team affiliation for a player who has appeared in an official match for a youth national team — as long as the player had dual citizenship at the time.
"We are absolutely thrilled," U.S. coach Jurgen Klinsmann said in a statement on Tuesday. "He is a very special talent. We wanted him to feel comfortable with our program and listen to his heart when making this decision."
Green trained with the U.S. squad in Germany this month ahead of its friendly against Ukraine.
"Obviously this was a big decision, and I spent a long time discussing it with my family," Green said in a statement issued by the U.S. Soccer Federation. "I was born in Florida and my father still lives there, so I have deep roots in the U.S. I'm very proud to be representing the United States.
"A big part of the decision was the experience I had in Frankfurt. All the players were super nice and welcomed me from the beginning. Clint Dempsey gave me a jersey with my name on it, and the way they supported me gave me a lot of belief. The coaches have shown a lot of trust in me, and now I hope to do everything I can to earn a spot on the World Cup roster."
Green has been invited to the U.S. training camp ahead of an April 2 friendly against Mexico in Glendale, Arizona.
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