The 34-year-old was a free agent after leaving Chinese club Dalian Aerbin, and has joined until the end of the season with the option of an additional year.
"I'm happy to be back in Spain," UEFA.com quotes him as saying. "I spent five years here and it's fantastic to be returning to play in the Liga. Valencia are a great team and a side who were always difficult to play against."
Keita previously spent five seasons playing in La Liga, with spells at Sevilla and Barcelona.