The imposing centre-back has signed a contract until 2014 with the Brazilian side, with the option to increase his stay for a further season should he wish to
According to the club's official website, the 33-year old, who has represented Brazil at the last two World Cups, will wear the No.18 shirt next season.
Should the move work out, the former Bayer Leverkusen man has the option to sign a one-year extension.
The move came as little surprise, after the player's father, Roberto, confirmed in an interview with Radio Guaiba that Juan would be joining the Brasileiro side.
"[Juan] is very happy to have agreed [a deal] with Inter. Flamengo had proposed, but he chose Inter. On Tuesday he will come to Porto Alegre," said Roberto.
Juan played 117 times during his five-year stay at the Stadio Olimpico, but struggled to hold down a place in the first team last season, making just 11 appearances over the course of the campaign.
The defender is not the first big-name signing at the club, with veteran Uruguay international Diego Forlan signing a three-year deal earlier in July.