The 22-year-old becomes the Magpies’ fourth signing of the summer after Matt Targett, Nick Pope and Sven Botman.
Newsonline reports that, the 22-year-old becomes the Magpies’ fourth signing of the summer after Matt Targett, Nick Pope and Sven Botman.
Isak scored 44 goals in 132 appearances for Sociedad, and has nine in 37 caps for his country.
“It was a big, big decision for me but it is a decision I am really happy with,” he said.
“It is a great, historical club that I think anyone would like to play for, with amazing fans. They were a big part of why I came here – and also the club’s project, which is one that I really believe in.
“You always want to go into a team and know how they play. I think Newcastle play very attractive football and I want to be a part of that.”
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Isak’s arrival at St James’ Park boosts Newcastle’s attacking options given the injury record of Callum Wilson, who is currently out with a hamstring strain.
Newcastle boss Eddie Howe said: “I’m very excited that Alexander has chosen to join us. He is an outstanding talent who is capable of making a big contribution to the team.
“He is technically very good, with a mix of attributes that will complement us, but he is also at a great age to grow with this club.
“I want to thank the club’s owners and transfer committee for bringing Alexander to Newcastle. We are all delighted to have him on board.”