AS Roma's Brazilian midfielder Mancini has become Inter Milan boss Jose Mourinho's first major signing, the Italian champions said on Tuesday.
Roma, who finished second to Inter last season, announced an initial agreement last week and the deal has now been finalised after the player returned to Italy from holiday.
Inter said in a statement the skilful and versatile Mancini, who normally operates on the left wing, would be presented to the media later but no financial details of the transfer were given.
The 27-year-old, who had an uneasy relationship with Roma coach Luciano Spalletti, is the second top player in two seasons to switch between the two 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.