The deal between Inter and the striker, who left Portugal after seeing out the end of his contract, was reached earlier this year, and was confirmed after he completed his medical.
Taremi made 182 appearances for Porto after joining in 2020, scoring 91 goals in all competitions.
The Iran international enjoyed a distinguished four-year spell at the club, winning the Primeira Liga, two Portuguese Super Cups, three Portuguese Cups and the Portuguese League Cup.
Taremi is the third player to move to San Siro so far this transfer window, joining Piotr Zielinski and Josep Martinez at the club.