The Chile international, who previously worked with Inter head coach Antonio Conte at Juventus, had spent the past two seasons at Camp Nou.
Vidal featured 43 times for Barca last season but was deemed surplus to requirements by new Blaugrana boss Ronald Koeman and is the third midfielder to depart this window, along with Ivan Rakitic and Arthur.
Inter confirmed the signing on their official website and are reported to have paid €1million plus add-ons for the 33-year-old.
Vidal, who was into the final year of his Barca contract, has also previously represented Colo-Colo, Bayer Leverkusen and Bayern Munich.
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