The Brazilian signs for an undisclosed fee after six years at the Italian club, allowing the Reds to loan out some of their young goalkeepers to get some experience elsewhere
The 31-year-old, who had been expected to join the Reds, finally put pen to paper having completed his medical.
He will put pressure on current number one Pepe Reina, who has been a regular at the club since signing from Villarreal in 2005 for £6 million and has been rumoured to be a target for other clubs in the past.
Doni made 179 appearances for Roma in his six-year stint in Italy, twice winning the Coppa Italia alongside a SuperCoppa Italiana.
The Brazilian international has represented his country on 10 occasions and was part of the squad who won the 2007 Copa America.
Doni is one of many to have signed for Livepool this summer.
He follows Jordan Henderson and Charlie Adam into Anfield, with Stewart Downing looking set to join them.
The stopper began his career in his native Brazil with Cruzeiro before moving to Juventude in 2005
He only spent a few months with the South Americans, before moving to Italy to play for Roma.
Speaking to Liverpoolfc.tv, the club's Director of Football, Damien Comolli, said: "We are delighted to sign Doni. It's a position we felt we had to strengthen because it will allow our young goalkeepers to go on loan and get some experience at some point during the season, which is crucial in their development.
"We are getting a very experienced goalkeeper who is used to handling the pressure of big games and international matches with Brazil. He had a very good end of last season with Roma, so we are very pleased."