Dortmund have not disclosed the length of the deal but sources said Reyna, 18 on Friday, has signed a new deal that would last until 2024.
The young forward made his USMNT debut on Thursday against Wales and is regarded as one of the most promising talents in the Bundesliga.
Sources have told ESPN there was interest in Reyna from the Premier League and La Liga, but he was focused solely on continuing his career with Dortmund.
Reyna signed an initial three-year deal when he agreed to join Dortmund from MLS side New York City in November 2018. He arrived at BVB in July 2019 and went on to make his debut in February 2020.
He has been an ever-present this season, making 12 appearances for Dortmund, scoring twice, and contributing five assists.
"Right now, I am very, very happy here, and that is all that counts," Reyna said in September.
"Dortmund is not a stepping-stone to me. It is a club that is fighting for titles. The incredible fans alone make Dortmund something special."
Dortmund are also set to hand teenage centre forward Youssoufa Moukoko first-team action after he celebrates his 16th birthday on Nov. 20.
Reyna's debut for the USMNT was due to happen earlier this year, but the fixture schedule was interrupted due to COVID-19. Reyna has now followed in father Claudio's footsteps in playing for the USMNT.
ESPN's Tom Hamilton contributed to this report.