He found himself out of the Schalke team more this term, however, featuring just seven times in the league before the mid-season break.
Incoming ... pic.twitter.com/ETDcoHT42Y— AS Monaco EN (@AS_Monaco_EN) January 3, 2019
Monaco are enduring a woeful season domestically and look in real danger of relegation, having won the title under Leonardo Jardim in 2016-17.
Thierry Henry's men are second from bottom, with some suggesting their over-reliance on young players is backfiring.
No fewer than 19 players aged 22 or younger have featured for Monaco in Ligue 1 this term, but the move for Naldo suggests the club are now looking to older players in a bid to save them from relegation.
Naldo joins until June 2020 and Monaco are reported to have paid approximately €1million.