The 22-year-old, capped 37 times by Sweden, moves to the Premier League side on a five-year deal, bringing an end to three successful years with the La Liga outfit.
Signed from Borussia Dortmund, Isak scored 33 goals in 105 league appearances for La Real, the last of which came in a 4-2 defeat to Barcelona on August 21.
“I’m delighted to have it done. It was a big, big decision for me but it is a decision I am really happy with,” he told the club website.
“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.”
Isak becomes Newcastle’s fourth signing of the transfer window, following the acquisitions of Sven Botman, Matt Targett and Nick Pope.
They had been in the market for a striker since the end of last season, having previously been linked with Everton’s Dominic Calvert-Lewin as well as Isak.
Eddie Howe’s need for a forward might have been expedited, however, by an injury to Callum Wilson, who has sustained a hamstring problem.
Newcastle are unbeaten in the early rounds of the 2022-23 Premier League season, drawing with Manchester City and Brighton and Hove Albion after starting the campaign with a win against Nottingham Forest.
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