The 19-year-old joined Leipzig from Barcelona in August last year, but has failed to impress in Germany, making just six appearances across all competitions.
Valencia are 10th in LaLiga, seven points off the top four, and the club will be hoping that Moriba's signing provides the push needed for them to climb back into contention for European football.
"Now that I'm here, I have to perform to my best and give my all, in order to put Valencia CF where the club has always been," Moriba said.
Moriba, who has been capped by Guinea five times, came through Barcelona's famed La Masia academy, but a breakdown in negotiations over a new contract saw him leave the club in acrimonious circumstances.
He could face his former employers when Valencia host them in a league fixture on Feb. 20. ($1 = 0.8967 euros)
