Ajer wrote on his Instagram account: “Five years ago I travelled to Glasgow not knowing what to expect. The stories I got told seemed incredible, but the very first time I walked out at Celtic Park I truly understood how big a club Celtic really is.
“'Football without fans is nothing', and I want to thank every supporter that has contributed to making my time at Celtic so special.
“I also want to thank the managers that have developed me as a player, back room staff that has helped me through ups and downs, and also my teammates that have become great friends that will last a lifetime.
“A special mention also to John Kennedy, the most important person for me during my time at Celtic. Without him, we would never win the quadruple treble."