The Dutchman will remain at Newcastle until the summer of 2018 but will spend the remainder of the campaign with Ajax.
Krul, who has won eight caps for the Netherlands, will replace Dutch international team-mate Jasper Cillessen at the Amsterdam Arena, after he joined Barcelona in a £12.8m deal.
The 28-year-old Krul has been out of action since October when he suffered a season-ending injury while playing for his country.
Matz Sels, who signed a five year-deal with the Magpies after joining from Belgian side Gent in June, has started all four of Newcastle's games in the Championship so far this season.
Elsewhere, teenage striker Adam Armstrong has signed a new four-year contract at St. James' Park.
The 19-year-old, who scored 20 goals while on loan at Coventry City last season, was called up for England's under-20 squad on Thursday ahead of their friendly against Brazil next month.