The Japan defender had his contract terminated a year early this season at Arsenal and has joined Ajax as a free agent.
"I'm really happy to be here," Tomiyasu told the club's official channels.
"I want to thank everyone who supported me. I was sidelined for a whole year; it was a very difficult time.
"Without their support, I wouldn't be here today."
He also said: . "I've worked hard and never given up. This club represents a new beginning for me.
"When I spoke to Ajax, I immediately sensed their faith in me. That's what convinced me."
The veteran also stated: "I can play centre-back or full back. It doesn't matter what position: I'll give my all in every game. After these twelve months, I know how hard I've worked to come back stronger. Now I feel ready."