The deal for the 34-year-old, who played for several French sides before joining Sevilla and then spent four trophy-filled years at Barcelona, is subject to a medical, Valencia said on their website.
A rangy defensive midfielder who is powerful in the air and has a keen eye for goal, Keita left Barca in July 2012 following the departure of coach Pep Guardiola, with whom he had a close relationship.
Guardiola, who is now at Bayern Munich, used Keita almost exclusively as a substitute and he helped Barca win three Spanish leagues, two Champions Leagues and two King's Cups.
He will give Valencia an extra option in midfield for their Europa League campaign - they play Dynamo Kiev in the last 32 next month - and as they seek to secure a place in Europe again for next season.
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- Pep Guardiola