Inter land Nagatomo on loan
"The champion of Asia arrives from Cesena, who have received Davide Santon as part of the deal, also on loan," an Inter statement said.
Inter had said their January recruitment drive was over after the arrivals of striker Giampaolo Pazzini, midfielder Houssine Kharja and defender Andrea Ranocchia but an injury to left-back Cristian Chivu and replacement Santon's poor form prompted action.
Nagatomo, who only joined promoted Cesena from FC Tokyo at the start of the season, played in Japan's 1-0 Asian Cup final triumph over Australia on Saturday.
His cross set up Tadanari Lee's stunning volley for the extra-time win of former Inter coach Alberto Zaccheroni's Blue Samurai.
Inter are fourth in Serie A after a pick-up in form under new coach Leonardo, who replaced Rafael Benitez on December 23.
The Spaniard departed after criticising the club for their lack of transfer activity, something they have now addressed.