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The full-back was due to see his previous deal expire at the end of the season, although it was widely expected fresh terms would be agreed.
Inter announced the news on Friday (AEDT), less than 48 hours after Darmian scored the winning goal in the Coppa Italia quarter-final against Atalanta.
"Hi Inter fans. I'm delighted to keep on wearing these colours. See you at the stadium," Darmian said.
Darmian joined Inter in 2020 from Manchester United, initially on loan before making a permanent move the following year.
He was part of the Serie A winning side in his first year at the club and lifted the Coppa Italia last season, as well as the Supercoppa Italiana – a trophy Inter retained last month by beating rival Milan.