5:27PM BST 15 Jul 2011
The Brazilian is thought to have joined on a free transfer after agreeing personal terms and passing a medical.
Liverpool's director of football Damien Comolli told the club's website: "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."
31-year-old Doni has played 10 times for Brazil and made 179 appearances in six seasons for Roma, with whom he twice won the Italian Cup.
Doni follows Jordan Henderson and Stewart Downing in agreeing to join Liverpool since the end of last season, when the club finished in sixth place following a resurgence under returning manager Kenny Dalglish.