The 34-year-old was a free agent after his Valencia deal expired and Roma have won the race to capture his signature.
Mali international Keita counts Barcelona, Sevilla and Lens among his former clubs and will add experience to a Roma squad preparing for a return to the UEFA Champions League and a Serie A title push.
A Roma statement confirmed the signing and read: "AS Roma would like to announce that an agreement has been reached for the signing of Malian player Seydou Ahmed Keita.
"The contract has the duration of one year, expiring on June 30, 2015."
Keita is likely to be absent for Roma during the Africa Cup of Nations, which is due to place in January and February next year in Morocco.
Mali have finished third at the last two Africa Cup of Nations events.
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