The 36-call Italian national emerged at the Rome academy in 2011, first as a midfielder, and made 280 appearances for Serie A club.
He was appointed captain in 2019, but spent part of last season on loan with Valencia, appearing 14 times in all competitions.
PSG technician Thomas Tuchel showed Wednesday that he is on a new right-back and now an agreement has been reached with Rome.
The French champions, connected to Hector Bellerin’s Arsenal, announced on Friday Florenzi’s arrival on his official website.
“I am proud and satisfied to become a Player of Paris Saint-Germain,” said the 29-year-old.
“This is one of today’s wonderful European clubs, demonstrated throughout their career in the Champions League final last month.
“I need to do as productive as possible for Paris Saint-Germain and the fans. We have normal demanding situations ahead of us this season.
“It will be an emotion and an honor to face them with my new teammates, who are among the players of the world. “
PSG, seven first-team players due to positive coronavirus tests, lost 1-0 to Lens on Thursday in their first attack of the 2020-21 campaign.