The Belgium international was a reported target for Arsenal's Premier League rivals Manchester United, but has now agreed a five-year-deal at Camp Nou.
Arsene Wenger confirmed Vermaelen was headed for the Arsenal exit door earlier this week, and Barcelona announced the transfer in a statement on their official website.
It read: "FC Barcelona and Arsenal FC have reached an agreement for the transfer of Belgian player Thomas Vermaelen.
"The player will sign [a] contract [at] Barca for the next five seasons."
Despite possessing the captain's armband at the Emirates Stadium, Vermaelen fell out of favour due to injury and lack of form last season, making just seven top-flight starts for Wenger's men.
The 28-year-old arrived in England from Ajax in 2009 for a reported fee of £10 million, and has since made 110 Premier League appearances.
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