Reyna, who celebrated his 18th birthday last week, progressed from Dortmund's under-19 team to link up with the senior side in January and has made 22 league appearances, scoring two goals with five assists.
"Gio's development in recent months has been incredible. He'll definitely be an important part of Borussia Dortmund's sporting future," the club's sporting director Michael Zorc said in a statement on their website.
Reyna, who also has Champions league experience, said he was delighted to continue his development at Dortmund.
"BVB are a big club that can challenge for titles and are in all the important competitions... I have already learned a lot in Dortmund and I want to learn a lot more in future," he said.
Barca paranoid Dembele 'has plan' to join United - Euro Papers
Dortmund are third in the Bundesliga standings after seven games this season, three points behind champions Bayern Munich, and visit Hertha Berlin in Saturday's late game.
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