Bourssia Dortmund and Real Madrid have both been fighting for the youngster who has been handled as the next Zlatan Ibrahimovic.
This is how the club has announced the positive news:
Hjärtligt välkommen, Alexander Isak! #welcomeisak #bvb Alle Infos: https://t.co/ZadYC8FTSX pic.twitter.com/GObv1G803Z
— Borussia Dortmund (@BVB) 23 de enero de 2017
According to rumours, the Bundesliga club paid €9 million for the Swede.