The 25-year-old has joined the Reds in a deal reportedly worth £20 million (€23.5m).
Aquilani, 25, is believed to be earning around £80,000 (€94,000) a week.
Aquilani has not played since March due to an ankle injury and is only expected to be fully fit in around four to eight weeks time.
Aquilani is Liverpool’s second major summer signing after England right-back Glen Johnson joined from Portsmouth.
Liverpool boss Rafael Benitez targeted the Italian on Wednesday just hours after the Reds sold Spanish midfielder Xabi Alonso to Real Madrid for £30m (€35m).
“Alberto has a winning mentality and great experience in both Serie A and the Champions League,’’ said Benitez.
Aquilani, has played 11 times for Italy and has been with Roma, the club he supported as a child, since he was 16, making his debut as an 18-year-old under current England boss Fabio Capello.
“Aquilani has long been recognised as a top-class talent in Italy, captaining his country at both U19 and U21 levels before establishing himself in the senior national side,’’ said Benitez.
The Italian had an operation on his ankle injury in May after struggling with injury problems which restricted him to just 14 appearances for Roma last season.
Aquilani is expected to be at Anfield today to witness his new team-mates take on Atletico Madrid in the club’s final pre-season friendly before next weekend’s big kick-off.
The Reds had also been linked with Wigan’s Lee Catermole and Valencia’s David Silva.
Last summer a move for then Aston Villa midfielder Gareth Barry — now at Manchester City following a £12m (€14m) summer transfer — fell through after Benitez was unable to sell Alonso to raise the sufficient funds for the deal.
Dirk Kuyt believes Aquilani can play a crucial role in helping bring Liverpool’s 19-year wait for a league title to an end.
“It’s good because we will be signing a new player with a lot of quality,’’ said the Dutch international.
“Xabi is a very good player but these kind of things happen in football. Teams are changing all the time. Xabi played very well for Liverpool but that’s all history now and we have to move on.
“We have to look forward. That’s what Xabi is doing with Real Madrid and we have to do the same.
“We have a lot of good players still here and we are all determined to go one better in the league after coming so close to winning the title last season.’’