The 21-year-old, who is 6ft 8in tall, began his career in Holland with second-tier club FC Emmen where he made over 30 senior appearances before joining Ajax in July 2019.
By then he had already made his first start for the Dutch under-21s, having also represented them at under-19 level, making his debut against an England team featuring Jadon Sancho and Phil Foden in September 2018.
He made four first-team appearances last season for Ajax including one in the Europa League.
Head coach Graham Potter said, “We’re delighted to welcome Kjell to the club. He’s a player we have been monitoring for a while and we see him as a player with a lot of the right attributes.
“As well as his size he is someone we think will work very hard to improve all aspects of his game as he gets used to English football.
“We wish him all the best and look forward to working with him.”