The Belgium international, who played 150 games for Arsene Wenger’s side in five seasons, moves to the Nou Camp for a undislcosed fee, widely believed to be £15m.
"Thomas Vermaelen is now officially an FC Barcelona player," confirmed the Spanish giants.
"A few minutes past 11:15am, the player signed a contract with the Catalan club. It is an agreement that binds him with the club for the next five seasons, until 2018/19."
"I feel very happy," Vermaelen told Barca TV. "I'm very excited to play for this beautiful club; it's one of the best in the world.
"It's a dream of a lot of people to play for this club, you play with the best players in the world and I'm very honoured to play with them in this team.
"I expect pressure like always; I played for Arsenal as well and it's not different from this club, you need to win every single game, so I understand there will always be pressure to deliver a good performance.
"My first goal is to adapt as quickly as I can; it's a different country for me, a different language as well and coaches, so I need to adapt and see how far we can go."
The fee is undisclosed for a player who had been club captain for the Gunners for the past two seasons but was frequently on the bench last term, with Per Mertesacker and Laurent Koscielny the first-choice pairing at centre-back.
He made only 13 starts in all competitions and 21 appearances in total for Arsenal during the campaign, which concluded with him being an unused substitute in the Gunners' FA Cup final victory over Hull City as they collected their first trophy in nine years.
Vermaelen was initially an integral part of the north London club's defence after signing from Ajax in 2009, but form and fitness subsequently put his starting spot in jeopardy.
The 28-year-old had been linked with Manchester United, but it is understood that Arsenal proposed a swap deal which the Red Devils were not prepared to do.
While indicating on Thursday that United were among the clubs to have made an offer for Vermaelen, Arsenal boss Arsene Wenger also said he would prefer any sale to be made to an overseas side.