Javier Pastore has ended a seven-year association with Paris Saint-Germain to join Roma.
Pastore, 29, arrived in Rome on Monday and underwent a medical on Tuesday before signing a four-year contract with the option for an extra year.
A source close to the Argentina international said Roma paid PSG €20 million and will pay up to another €4m in bonuses.
Pastore won 19 domestic trophies since joining PSG for a fee of €43m from Palermo in 2011, but also suffered injury setbacks.
He becomes Roma's ninth signing of the summer, hours after Nicolo Zaniolo and Davide Santon joined as part of a swap deal for Radja Nainggolan.
"I did not think that this day would ever come, but it is now time to say goodbye," Pastore wrote on Facebook.
"I would like to thank all of the Parisians, who welcomed me magnificently and made me feel at home for all of these years. ... Seven years is a very long time, yet it is also not enough, in a way, because I would have liked to have given you even more."
In a statement, PSG chairman and CEO Nasser Al-Khelafi thanked Pastore for his dedication to the club.
"I will never forget the strong bonds that tie Javier, our first major international signing, to all those who love our club," Al-Khelafi said. "Since his debut wearing our jersey, Javier's superb technique and visionary passes enchanted the Parc des Princes crowd, who have always reserved a special place in their hearts for the most elegant of players.
"Javier will remain one of those players who shaped the identity of our team, our club, our project, both on a national and international level."
Justin Kluivert, Marco Tumminelli, Antonio Mirante, Ante Coric, Ivan Marcano and Bryan Cristante have also joined Roma, with Lukasz Skorupski joining Nainggolan in leaving.
"The arrival of Pastore delights me because we're talking about a player who is capable of enchanting the fans with his skills," Roma sporting director Monchi said in a statement. "During my career as a sporting director, I have signed lots of players and Javier is probably the one with the greatest quality."
Pastore's move also helps PSG with their short-term UEFA financial fair play concerns, adding to the money already brought in by the sale of Odsonne Edouard to Celtic as the target of between €40m and €60m nears.
ESPN FC Italy correspondent Ben Gladwell contributed to this report.