The Guinean midfielder is back in Spain only half-a-season after leaving Barcelona for the Bundesliga.
A physically strong and technically skilled central midfielder, he made just six appearances for Leipzig totalling 100 minutes.
He worked under the instructions of Jose Bordalas, although Jasper Cillessen, Omar Alderete and Gabriel Paulista were all missing through different muscle injuries.
Ilaix is the second player Valencia have recruited so far this January transfer window after centre-back Eray Comert.
They’ve also been linked with a move for former Sevilla man Bryan Gil, who like Ilaix has failed to settle after a summer move to Tottenham Hotspur.
Valencia are currently tenth in La Liga. They’re five points off a European spot and sixth-placed Real Sociedad and seven behind fourth-placed Atletico Madrid and the Champions League.