Roma signed English defender Chris Smalling on a permanent deal after a successful spell on loan from Manchester United, the Premier League club confirmed on Monday.
The 30-year-old scored three goals in 37 appearances in all competitions last season as Roma finished fifth in Serie A.
“Manchester United have reached an agreement with AS Roma for the transfer of defender Chris Smalling,” United said in a statement.
“The centre-back was signed by Sir Alex Ferguson in 2010 from Fulham, and went on to make 323 appearances for the club, scoring 18 goals.”
Roma have reportedly paid 15 million euros (£13.5 million, $17.5 million) plus a further five in add-ons for Smalling.
Earlier on Monday, United made their second signing of the window as Brazilian international left-back Alex Telles joined from Porto.