Due to the injury record of Callum Wilson, awaiting the results of a hamstring scan, the 22-year-old Sweden international would improve the offensive options for the Magpies.
Isak, a former footballer for Borussia Dortmund, with nine goals in 37 appearances for his nation and 44 goals in 132 games for Sociedad.
Newcastle’s record transfer was Joelinton, who they acquired for £40 million in 2019.
The Brazilian striker, now switched to midfield, was acquired from Hoffenheim in Germany.
Last January, the club also spent £35 million plus add-ons for Bruno Guimaraes, a fellow Brazilian midfielder.
Still, the acquisition of Isak would signal a dramatic change under Newcastle’s new owners, who took over the club in October and have adopted a systematic approach to the transfer market.
Following the acquisitions of Matt Targett, Sven Botman, and Nick Pope this summer, the addition of Isak would almost treble Newcastle’s expenditure this summer to just under £120 million, bringing the club’s total spending during the new owners’ ownership to over £200 million.