Kristoffer Vassbakk Ajer (born 17 April 1998) is a Norwegian footballer who plays for Scottish Premiership club Celtic. Ajer has previously played for Lillestrøm and Start in the Tippeligaen.
Ajer was born in Norway and he started his career with Rælingen FK before joining Lillestrøm when he was 10 years old.
Ajer joined Start in 2014 as a result of his parents moving to Kristiansand. He made his debut for Start on 19 July 2014 in a 2–1 win against Bodø/Glimt.
At 16 years of age, Ajer captained Start in a 1–1 draw against Lillestrøm on 7 April 2015, becoming the youngest ever footballer to captain a team in the Tippeligaen.
In early January 2016, Ajer had a one-week trial with Scottish club Celtic. On 17 February 2016 Ajer signed a four-year contract with Celtic and joined the club in June 2016 for a fee which could rise up to £650,000.He was loaned to Kilmarnock in January 2017.