AS Roma's Brazilian winger Mancini has become Inter Milan boss Jose Mourinho's first major signing, the two clubs said on Wednesday.
Roma, who finished second to Inter in Serie A last season, announced an initial agreement last week and the deal has now been finalised after the player returned to Italy from holiday.
The skilful and versatile Mancini has moved for 13 million euros ($NZ25.3 million), Roma said in a statement. They added that a friendly between the teams for the season after next was part of the deal.
"I thank Roma for everything they have done for me, for how they have treated me and for the way in which they have behaved in the negotiations with Inter," Mancini told a news conference.
The 27-year-old, who spent five years at Roma and had an uneasy relationship with coach Luciano Spalletti, is the second top player in two seasons to switch between the rivals.
Romania centre back Cristian Chivu left Roma for Inter at the start of last season in an acrimonious move.
Inter were tracking Mancini even before Mourinho's arrival and his signing now makes it less likely they will bid for Porto's Ricardo Quaresma.
Their interest in Chelsea midfielder Frank Lampard remains, however.
Luis Figo, a rival to Mancini on the left flank, is still expected to sign a new one-year deal with Inter rather than retire.
Mancini is no relation to former Inter coach Roberto Mancini, who was sacked at the end of last term despite Inter retaining the league title.
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