Guarin, formerly of St Etienne, comes as a direct replacement for Sulley Muntari, who has left the Nerazzurri on loan to rivals Milan.
Guarin, who joined Porto in the summer of 2008, has moved for a loan fee of 1.5 million Euros, with the option to buy after six months.
He scored 10 goals last season as Porto won their domestic championship and the Europa League but would have found opportunities limited after they signed Argentina international Lucho Gonzalez from Marseille.
Another midfielder has arrived at Inter in the shape of Italy international Palombo. The 30-year-old has signed for the club from Sampdoria until 2015.
"I am very pleased to be joining Inter, a great team," he told the club's official website.
The Italian club also confirmed the signing of young Brazilian defender Juan from Internacional.
Juan, 20, won the World U20 Cup in Colombia and the South American equivalent in Peru. He has also played 25 times for Internacional's senior team both in domestic and continental competition.
Primarily a centre-back but able to play on the left side of defence, he signs on a four-and-a-half-year deal.