The Norwegian becomes Lee Clark’s fifth signing of the January transfer window, following on from Karleigh Osborne and the Newcastle trio Cal Roberts, Freddie Woodman and Sean Longstaff.
Ajer, 18, moved to Celtic in June from IK Start in his homeland, but was limited to one first-team appearance, a 30 minute cameo against Lincoln Red Imps in the Champions League qualifiers.
Clark, speaking to the club’s official website, said: “Kris is a very versatile player, who can play for us in many positions. We have fought off a lot of competition to secure the services of Kris and we are delighted to have a player of his quality here at the Club.”
The 6ft 5in teenager can play in defence and midfield and has been capped for his country’s under-19s. He made his first-team debut at the age of 16.