The 34-year-old Mali international completed his medial on Friday morning and will immediately join up with his new side.
Valencia said in a statement on their official website: "Seydou Keita completed his medical examination to become player of Valencia CF until June 30, 2014, with an option for another season this morning of Friday, January 31.
"The event was held at medical facilities Ciudad Deportiva de Paterna, near Divina Pastora Seguros, and he will immediately join training with his new team mates."
Keita started his career in France with Marseille, Lorient and Lens before moving to La Liga with Sevilla in 2007.
He spent one season with the Andalusian club before Barca paid a reported 14million euros to take him to the Nou Camp.
During four seasons at Barca, the midfielder won a host of trophies, including three Primera Division titles, two Champions Leagues and two Copa del Reys, before leaving the Catalan club in 2012 to join Chinese outfit Dalian.
He had one year left on his contract at Barca but activated a clause that allowed him to leave should he not start more than half of Barca's fixtures in the previous season.
Valencia are 10th in the Primera Division standings on 25 points, 11 adrift of the top six.