18-year-old winger Williot Swedberg has now been presented by the La Liga club.
Who is said to pay 4.7 million euros for the transition, corresponding to 49 million kronor.
That makes him the most expensive player Hammarby sold, three million more than Akinkunmi Amoo that FC Copenhagen bought last winter.
And Swedberg will be the Allsvenskan's sixth most expensive export.
- I have had fantastic years in Hammarby and now I look forward to a new challenge, Swedberg says to the club's website.
The 18-year-old has this year accounted for five goals in ten Allsvenskan matches.
The text is updated.