The 18-year-old, who is the third-youngest player at Euro 2016, signs a five-year deal for an undisclosed fee.
On Monday, Dortmund signed Bayern Munich midfielder Sebastian Rode, 25.
Last week, they agreed a deal for Barcelona's Spain defender Marc Bartra, 25, after meeting his £6.25m release clause and previously signed Rennes forward Ousmane Dembele, 18.
The Rode and Dembele deals were completed for undisclosed fees.
Find all the latest football transfers on our dedicated page.

The 22-year-old, who is on a season-long loan at Turkish side Kasimpasa from Chelsea, has been told by the team doctor he needs time to recover.
"I feel sad about it as it was another good opportunity for me to represent my country," Omeruo told BBC Sport.
"The doctor insists I need rest to start next season without injury."
Omeruo was first called up by Nigeria in January 2013 and he went on to play at that year's Africa Cup of Nations and Confederations Cup - and at the 2014 World Cup.
He admitted he was looking forward to playing at the Games, especially after the Super Eagles' failure to qualify for the 2015 and 2017 African Cup of Nations.
"It is disappointing and frustrating to say the least. I'd be lying if I said I would not want to be out there playing," he said.
"But at the same time I need to be fully fit to be at my best and I have no doubt that the squad will do great in Brazil.
"I would also like to thank the coach [Samson Siasia] for even considering me in the first place."
A firm fixture in the Kasimpasa side, such has been Omeruo's impact that the Turkish club are now exploring the option to buy him at the end of his loan spell.
He joined Chelsea from Belgian side Standard Liege in 2012 and developed on loan at ADO Den Haag in Netherlands in an 18-month spell from 2012 to 2013.
The centre-back, who is yet to make his competitive debut for Chelsea, then went on loan to Championship side Middlesbrough in January 2014 and made 14 appearances.
He returned to Teesside again for the whole of 2014/15 season where he made 22 appearances.

The Argentina forward, 28, is serving a three-match ban for his violent conduct in City's 3-1 win against West Ham.
Aguero has scored three goals in three Premier League matches this season, but Mourinho says his absence makes Pep Guardiola's side "unpredictable".
"He doesn't play and they have amazing options," said Mourinho.
"It is more difficult. When Sergio Aguero is available he plays, we know their formation."
Mourinho suggested any one of Kelechi Iheanacho, Raheem Sterling or David Silva could start as a "false nine" in the derby at Old Trafford.
"We have to reduce the unpredictability," added the Portuguese.
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The Spaniard, whose opening round in Munich contained six birdies, trails leader Wade Ormsby of Australia by two shots and Belgian Thomas Detry by one.
England's Tommy Fleetwood, who finished fourth at the US Open on Sunday, is tied for sixth, three strokes adrift.
Defending champion Henrik Stenson of Sweden also carded a five-under-par 67.
Fleetwood's compatriots Richard Bland, James Morrison and Matthew Southgate matched Fleetwood and Stenson's score.
"I felt like I played quite well, but not spectacular," said Garcia, whose Masters triumph at Augusta in April was his first major title.
"There's still three more rounds to go, so we're going to keep playing hard and hopefully we'll be up there on Sunday."
Borussia Dortmund have made FC Nordsjaelland and Turkey winger Emre Mor their fourth summer signing.
[NEXT_CONCEPT]
Nigeria international defender Kenneth Omeruo has been declared unavailable to play at the 2016 Olympic Games in Rio because of a groin problem.
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Manchester United manager Jose Mourinho says it will be "more difficult" facing a Manchester City side that does not include Sergio Aguero on Saturday.
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Sergio Garcia shot a six-under-par 66 to share third place at the BMW International, his first event in Europe since winning the Masters.