The 25-year-old came through the youth system at Real Madrid and was at the club from 2010 to 2016, however Mascarell only managed to make one first team appearance during his time there.
The La Liga side will reportedly pay four million euros for the Spanish national, a lot less than the market price of the player.
Mascarell has impressed the Spanish giants with his performances in the Bundesliga with Eintracht Frankfurt. And with multiple sides interested in the young star, Real Madrid have insured that they snap him up before the likes of Borussia Dortmund and Schalke swoop in.
The midfielder is expected to spend pre-season in the Spanish capital and that will be his chance to prove himself to the Madrid coaches. With the World Cup in the summer, the majority of the first team will be not present at the training ground, allowing Mascarell the opportunity to showcase his skills.
If things don't work out for the 25-year-old, there are reportedly a number of Bundesliga sides interested in signing the Spanish star, and Mascarell is thought to be more than happy playing in Germany.