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The Sweden striker has scored 26 league goals in Spain since arriving from Borussia Dortmund, while he also found the net seven times during a successful Copa del Rey run in the 2019-2020 campaign that ended the club's 34-year trophy drought.
After helping his country reach the last 16 at Euro 2020, Isak has committed his long-term future to La Real, who reached an agreement with Dortmund over a buy-back clause earlier in the offseason.
According to reports in the Spanish media, the new deal for the 21-year-old – who has been linked with a move to the Premier League – sees his release clause rise to €90million.
"I'm very happy to continue at La Real. It is a great club, I feel loved here and hope to enjoy things with this team for many more years," Isak said.
Real Sociedad finished fifth in LaLiga last term, good enough for Europa League qualification.
Isak scored 17 times in the domestic competition and while unable to grab a goal for his country during Euro 2020, his performances helped Sweden top a group that included Spain, Slovakia and Poland.
Its run in the tournament came to a dramatic end at Hampden Park on Tuesday, however, as a last-gasp winner at the end of extra time saw them go down 2-1 to Ukraine.