Neither of the Spanish giants will be allowed to sign players in the next two transfer windows and have been handed fines by world football's governing body.
FIFA have banned Real Madrid and Atletico Madrid from signing players for one year.
The Spanish giants were found guilty of breaching rules relating to the international transfer and registration of players under the age of 18.
In a statement released on Thursday, FIFA said: "The two clubs were found to have violated several provisions concerning the international transfer and first registration of minor players as well as other relevant provisions with regard to the registration and participation of certain players in competitions."
Atletico were fined 900,000 Francs (€822,000) and their city rivals were fined 360,000 Francs (€328,000).
The bans from football's world governing body mean neither club will be able to sign players for the next two transfer windows until January 2017, but will be allowed to sign and sell players in the current January window.
Atletico released a statement condemning the decision, vowing to lodge an appeal."Our club does not agree with the sanctioning ruling from the international football body and is studying the documentation received to lodge an appeal," the statement read."
Real Madrid released a longer statement with the same sentiments, claiming they never used foreign underage players without registering them with the Spanish football association. They also denied signing underage players without complying with Spanish FA procedures.
The Spanish top division is no stranger to such sanctions after Barcelona also had a transfer ban imposed on them in 2014 for the same offense.