- Swedish teen joins Dortmund
- Isak is nation's youngest goal scorer
The Sweden international, who was a rumored target for European champion Real Madrid, has signed a long-term contract with the eight-time Bundesliga winner.
"Alexander Isak is a hugely-talented striker, who many top clubs in Europe wanted to sign," Dortmund sporting director Michael Zorc said in a club statement.
"We are delighted that he has chosen Borussia Dortmund. Both BVB and the player himself are convinced that this transfer has great potential."
As he is a minor, Isak's transfer from AIK Solna still has to receive clearance from world football's governing body FIFA, although no problems are anticipated.
Isak, who was born in Sweden to Eritrean parents, became the Scandinavian country's youngest ever goalscorer in this month's 6-0 victory over Slovakia.
He made his debut for AIK in February 2016 and helped the club finish second in Sweden's top division.
, Sweden's most famous footballer, left his first club Malmo in 2001 to embark on a career which has taken him to some of Europe's greatest clubs, including Juventus, Barcelona and now Manchester United.