Real Madrid have agreed to a €30 million ($33 million) purchase of Theo Hernandez from Atletico Madrid.
Real Madrid are paying €6 million over his buyout clause.
The two Madrid teams have typically avoided doing business with each other. Atleti tried to move Theo to Barcelona, Liverpool and an unnamed German club after they were unable to agree upon a new deal with him.
Theo is a 19-year-old left back currently on loan with Alaves. Theo joined Atleti's academy in 2007.
Marcelo has been one of the world's best left backs over the past few years, but he turns 29 this week.