It is second defensive addition of the transfer window by Barcelona. Last month they confirmed the signing of Valencia's French defender Jeremy Mathieu for €20m.
Barca did not publish any financial details on their website but Spanish media said they had agreed to pay about 19 million euros for Vermaelen.
The 28-year-old defender joined Arsenal in 2009 after a nine-year stint at Ajax Amsterdam and quickly became a fan favourite while also being promoted to club captain.
Vermaelen's career has been hampered by injury and he made only 21 appearances in the 2013-14 season after falling behind Laurent Koscielny and Per Mertesacker in the pecking order.
Barca's defence was a weak point last season, when they failed to win major silverware for the first time in six years, and coach Gerardo Martino was replaced by former Barcelona and Spain midfielder Luis Enrique.
Vermaelen will compete for a place with Spain international Gerard Pique, Frenchman Mathieu, who joined last month from Valencia (who can also play at left-back), and Barca academy graduate Marc Bartra.
Javier Mascherano has mostly played in central defence since joining Barcelona from Liverpool in 2010 but after his performances in midfield for Argentina at the World Cup he is widely expected to feature there for the club next term.
Barca start the La Liga campaign, when they will be chasing a fifth title in seven years, at home to Elche on August 24.
If Arsenal have a plan in place to replace Vermaelen and fast, this should not be a bad thing at all for the Gunners. Truth be told, he hasn’t exactly been brilliant over the years for them due to injuries and mixed form.
They have got most of the money they have paid for Calum Chambers back who looks like he could be a hungrier and fresher option at the back. However, they could still do with using the money to bring in a more experienced alternative option otherwise they will miss his leadership qualities on more testing occasions.
SPANISH VIEW (Jorge Ordas, Eurosport Spain)
One of the big problems for Barcelona in recent years had been in central defence. After Puyol's retirement, Mascherano was playing there but it was not the best solution.
Now they have signed two defenders, Mathieu and Vermaelen but neither of them looks to have the quality to be the leader in Barcelona’s defence.
We know why Luis Enrique wanted them but, for the moment, we don’t think they will send Pique to the bench.
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