Chris Smalling has joined Roma from Manchester United in an £18m deal.
After weeks of negotiations, the clubs held positive talks in the past days and a deal worth €15m plus €5m in add-ons was eventually struck.
Smalling, who has signed a three-year deal at the club with the option of an additional year, had been pushing for a move to Roma throughout the summer, where he impressed last year on a season-long loan.
"Nothing in life worth having comes easy," Smalling told Roma's club website.
"My head and my heart has always been in Rome. I am delighted to now be yours. Daje Roma!"
Smalling also issued a farewell message to Manchester United supporters and has claimed that although he joined Roma on a temporary basis last year, he "instantly" knew it was going to be his long-term home.
"It all began in July 2010, and now 10 years, 323 appearances, 2 titles, and 6 cups later it has come to an end," Smalling said on Twitter.
"Utd is a special place, and we achieved special things together, something I am beyond proud about. An unrivalled culture, where winning is not a want it is a need.
"I leave not only a better player, but a better person, with Utd forever in my DNA. For that I have to thank all the staff, everyone connected to the club, and you the fans.
"Nothing surer you will continue to greatness. I arrived [at Roma] last summer, and although temporary at the time, I knew instantly it would be my permanent home. Cannot wait to get started again, the journey continues, enjoy the ride!"
The 30-year-old had been frozen out at Old Trafford following his return, training on his own and was omitted from all of United's matchday squads this season.
United had eight centre-backs on their books including Axel Tuanzebe, Phil Jones, Marcos Rojo and academy product Teden Mengi - with Smalling not part of boss Ole Gunnar Solskjaer's plans.
"Everyone at Manchester United would like to thank Chris for his years of service, dedication and professionalism, and wish him all the best for his future," United said in a club statement.
Smalling was Roma's top target this summer and the Serie A side have approached United multiple times in pursuit of a permanent deal but their offer fell short of the £20m asking price.
Inter Milan and Newcastle were also interested in the centre-back this summer after his three goals in 37 appearances for Roma last year.
Smalling returned to Old Trafford in August after the clubs were not able to find an agreement to extend his loan to cover the Europa League finals, where Roma suffered a round-of-16 defeat to winners Sevilla.
"We are pleased to be able to welcome Chris back to the club, as one of the pillars of our squad," Roma chief execuitve Guido Fienga said.
"I would like to thank Manchester United for the consideration they have shown throughout this strenuous and lengthy negotiation."