The Belgian international has long been linked with a move to the Emirates Stadium, and has now become manager Arsene Wenger's first signing of the close-season.
The 23-year-old has made more than 80 league appearances for the Dutch giants since joining in 2003 - scoring against Aston Villa in last season's UEFA Cup - and has established himself as one of the most highly-rated stoppers in the Eredivisie.
At Ajax, Vermaelen won the Dutch championship in 2003/04, and the Dutch National Cup and Dutch Super Cup both in 2006 and 2007.
“Thomas is a great signing for Arsenal Football Club, and we are all delighted that he is joining us," Wenger told the club's official website.
"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.”
While Vermaelen added: “Firstly, I have to say that I’m sad to be leaving Ajax and I would just like to thank everyone for their help and friendship towards me over the past eight years.
“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.
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