Wijnaldum underwhelmed in his first season in the French capital after joining on a free transfer from Liverpool last summer.
The 31-year-old made 38 appearances in all competitions, scoring three goals but rarely played the full 90 minutes.
New PSG manager Christophe Galtier considered Wijnaldum dispensable after adding the likes of Vitinha and Renato Sanches to his midfield options.
PSG's loss has become Roma's gain as the Europa Conference League holders have enticed Wijnaldum to the Italian capital to join the likes of Nemanja Matic, Mile Svilar, Zeki Celik and Paulo Dybala in moving to the Stadio Olympico this summer.
Jose Mourinho's side have the option to make the move permanent at the end of the season.
“It’s a really good feeling to be a Roma player,” Wijnaldum said.
“Everyone I have spoken to has given me great feedback about the club and its fans.
“The club made it clear just how much they wanted me in the efforts they made to complete the deal, which always gives a player a lot of confidence and belief. The reception from the supporters, on social media and elsewhere, has also been fantastic.
“I promise to give 100% and help the team compete for all our targets this season.”
Roma's general manager Tiago Pinto added: "Georginio’s decision to come to Roma is yet another example of players of the highest level, both in terms of their qualities on the pitch and off it, believing so strongly in the club’s plans that they put all their energy into ensuring that negotiations reached a positive outcome... We can not be anything other than delighted at his arrival.
"Gini brings qualities that will further enhance the competitiveness of our squad and a natural leadership that has seen him become captain of one of the best national sides in the world."
Wijnaldum has won 86 caps for the Netherlands national team.
