The malian midfielder joins as a free agent after a six-month stint in Spain with Valencia, who he helped reach the semi-finals of the Europa League.
The 34-year-old had been linked with a possible move to the English Premier League.
AS Roma, coached by Rudi Garcia, finished runners-up in Serie A last term and will play in next season's Champions League.
Keita joined Valencia in January until the end of last season on a free transfer from Chinese side Dalian Aerbin.
He won 14 trophies in four seasons with Barcelona between 2008 and 2012.
