The club announced on their website and social media feeds on Tuesday that the Argentine midfielder had joined the club.
Pastore has signed a deal that runs until 2023, and returns to Italy having played there for two seasons with Palermo before moving to PSG in 2011.
“I am very happy to be here at Roma,” he said, speaking to the club’s official website. “I hope to repay in full all the enthusiasm, support and affection that Roma fans have shown me already.”
The new signing has already met the club’s top brass, such as Sporting Director Monchi and club legend Francesco Totti. He becomes the eighth addition to the club’s squad already this season, as they look to build on a Champions League run that took them to the semi-final, beating Barcelona on the way.