The Eredivisie club confirmed they landed the Standard Liege midfielder for a fee of €12.5 million (£11m/$14m), with Marin signing a five-year contract with the Champions League quarter-finalists.
The 22-year-old has spent the last two-and-half seasons with Standard Liege after a January 2017 move from Romanian club Viitorul.
He’s netted 14 times for Standard over his time at the club across all competitions.
Marin will be among the players who will be tasked will filling the shoes of De Jong, who is set to depart the club this summer for Barcelona for a €75m deal.
And the Romanian is aware of the comparisons between himself and the future Barca midfielder, though he believes they are two different types of players.
While De Jong flourishes in a deeper role, Marin styles himself as much more of an attack-minded player.
“I want just to be me,” Marin told the club’s website. “I saw that everyone compared me with Frenkie but I think we are two different styles of players because he’s more in front of the defence.
"He stays there, he has good quality of passes, he can eliminate [players] during the games.
“I’m more dynamic. I like to go into the opposition box, to score, to help my team how I compete.”
Marin is likely to be among many new faces at the club next summer as Ajax face battles to hold on to many of their top names.
Defender Matthijs de Ligt has been linked with moves to some of the biggest clubs in Europe, while players such as David Neres, Hakim Ziyech and Nicolas Tagliafico have also been reported as transfer targets for teams in the top five leagues.
While Marin is now a confirmed part of Ajax’s future, the club’s present including De Jong has them remaining in a battle for the Eredivisie title.
Ajax currently sit second, two points behind reigning champions PSV.
They also are set to battle Juventus in the quarter-finals of the Champions League, with the first leg to take place April 10 at the Johan Cruijff Arena.