Manager Arsene Wenger said: “Thomas is a great signing for Arsenal and we are all delighted that he is joining us. “Thomas is a solid performer with good experience of Champions League and UEFA Cup football and is also a regular with the Belgium national team.
“Thomas is primarily a left-footed centre back, but he’s multi-functional and can also play at left back.
“He is strong technically with a positive winner’s attitude and I have no doubt his arrival will strengthen our team. Thomas Vermaelen will be a huge asset to Arsenal.”
Vermaelen, who is 23 and was Ajax captain, commented: “I’m just so happy to be joining Arsenal. This is a great club with a world class manager, high quality players and a fantastic stadium.
“Everything is set up for Arsenal to be very successful for years to come and I joined this Club because I know Arsenal will be challenging for trophies.
“I have had a good taste of the Champions League during my time with Ajax and this is something I am looking forward to experiencing again with Arsenal. “I’m also looking forward to playing in the English Premier League, which for me, is the best league in the world. Also, I just want to assure the Arsenal fans that I will give everything for this club and I hope we can enjoy winning some trophies together next season.”