The Mali international, who was a free agent, signed a one-year contract with the Giallorossi on Thursday evening to become the club's first reinforcement for next season.
"Roma is a big club with big ambitions," Keita said to AS Roma Channel. "This offer has been on the table for three months so I have had the time to consider my options and make a well thought out decision."
The former Barcelona midfielder spent the second half of this season with La Liga side Valencia, making 18 appearances in all competitions.
The 34-year-old Keita, who was a reported target of Manchester United and Liverpool, believes he can make a big impact at Roma.
"With Barcelona I won a lot of trophies but I'm still hungry to win some more wearing this shirt," Keita said. "I will bring my experience, especially in the Champions League, even though there are a lot of strong players like (Francesco) Totti and Gervinho.
"I have already seen Roma play and it seems to me that they are a great team, you don't come second in Serie A by chance.
"I hope to give my contribution in terms of goals and assists in order to make the fans happy."
Roma, coached by Rudi Garcia, finished runners-up in Italy's top flight this season and will compete in the Champions League next term.