The Norwegian defender joins the English Premier League newboys after an improved second offer was accepted by Celtic who turned down Brentford’s initial approach.
Ajer had less than one year left on his contract and made it clear he wished to leave the East End of Glasgow after five successful years with the Hoops.
The 23-year-old joined from Start in the summer of 2016 and was quickly converted from an attacking central midfielder into a centre-back, which he refined in a loan move to Kilmarnock.
In total he made 170 appearances, becoming a key member in three of Celtic’s four consecutive trebles and even picked up the club’s Player of the Year from the disappointing 2020/21 campaign.
Celtic said in a statement: “Everyone at Celtic Football Club wish Kristoffer every success in his future career down south and thank him for his role in the club’s success in recent years.”