The son of NYCFC sporting director and former U.S. star Claudio Reyna, the 16-year-old midfielder will begin his Bundesliga career with the under-19s.
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''Giovanni Reyna is the son of former Bundesliga player Claudio Reyna. He plays in central midfield and is without doubt a huge talent," Dortmund academy director Lars Ricken said in a team statement.
Dortmund also announced the signing of Switzerland youth international forward Bradley Fink who joins from Swiss side Luzern.
"We're delighted that they've opted for Borussia Dortmund. Everyone at BVB is convinced that these two transfers show great promise,'' Ricken added.
Reyna most recently played a starring role at the CONCACAF Under-17 Championship this spring, scoring six goals while leading the U.S. to the final where they lost to Mexico.
He will look to emulate the success of U.S. international midfielder Christian Pulisic, who also joined Dortmund at age 16 in 2015.
Reyna has a Portuguese passport because of his paternal grandmother's ties to the country, allowing him to avoid having to wait until his 18th birthday to move abroad as other Americans must under FIFA regulations.
Reyna is considered one of the top players in the U.S. U17 pool, and he led NYCFC's U18/U19 team to the U.S. Development Academy championship. According to U.S. Soccer's website, he scored 15 goals in 22 appearances during the 2017-18 season.
His father, who earned 112 caps for the U.S. national team from 1994-2006 and was named to four World Cup squads, also played in Germany with Bayer Leverkusen and on loan with Wolfsburg.
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