Amrabat, 30, was allowed to leave Turkish giants Fenerbahce and had his contract terminated ahead of signing a two-year contract with Ajax.
"We are delighted to have secured Sofyan’s services. He is physically strong, possesses great footballing ability and brings a wealth of international experience," technical director Jordi Cruyff said on the club website.
"He also knows the Dutch league well, so we expect him to settle in quickly. In him, we have found exactly the type of player we had in mind."
Amrabat, a 78-time Moroccan international, previously played for FC Utrecht, Feyenoord, Club Brugge, Fiorentina, Manchester United, Fenerbahce, and Real Betis, where he spent last season on loan.
Ajax had been looking for a holding midfielder since Jordan Henderson's departure ahead of the 2025/26 season, which left a massive hole in the team. Youri Regeer has largely filled the role since, but is set to leave on loan to Werder Bremen.
The Netherlands-born Amrabat is delighted to join Ajax and has been in contact with Cruyff since April, he revealed.
"I got a text: ‘Do you still have this number?’ I had a look, saw Jordi’s profile picture and replied: ‘Jordi Cruyff? You never know these days.
"He called back straight away. We had a lovely chat, which lasted about half an hour. From that moment on, we’ve been in touch. I was quite surprised by his call. We’d been in touch once before when he was at FC Barcelona. A transfer didn’t go through back then."
After being contacted about joining Ajax, Amrabat was interested in the project in Amsterdam and was soon contacted by someone else from the club.
"Cruyff didn’t say, ‘You have to sign tomorrow.’ But he did tell me about his plans for Ajax. That made me curious. So we’ve always kept in touch. The coach (Michel Sanchez) called me afterwards as well. And that was positive too. It was very clear in every respect that Ajax wanted me. That’s very important to me. I’m a loyal person, and I’d like to repay that trust."
Amrabat's first potential test with Ajax will be a clash against reigning champions PSV in the Johan Cruyff ArenA - a game he's long looked forward to.
"I'm fit and really keen to play. I've been thinking about the Ajax v PSV match for a while now. I haven't played any matches recently, but I have been training. Ultimately, it's up to the manager to decide what he wants. But I'm ready."