The 19-year-old has joined the Black Cats from IFK Goteborg for an undisclosed fee and has signed a four-year contract.
Sweden Under-19 international Karlsson began his career at IFK Mariestad, but left for Goteborg, for whom he has made 10 appearances this season, in 2010 and picked up a Swedish Cup winners' medal last month.
He will go into the development squad along with fellow new boys Duncan Watmore and El-Hadji Ba.
- Sports & Recreation
- IFK Goteborg
- IFK Mariestad