The 19-year-old only made his senior debut last season while Racing were still competing in the Segunda División, but has since drawn admirers across the continent after helping the Cantabrian club secure promotion back to the top flight.
He turned down the chance to leave in the January window, choosing instead to see out the season in Segunda - a decision that culminated in the promotion he had been chasing.
Martínez will initially join the Castilla squad, with the possibility of training alongside the first team, continuing a path he began appearing on last season when he made ten Liga Hypermotion appearances for Racing Santander in the closing weeks of the campaign, scoring a memorable goal along the way.
Giving up a place in the top flight so soon after his debut and a standout goal was, by his own admission and those around him, the hardest part of the decision - but the pull of Real Madrid and the genuine possibility of a path to the first team proved decisive.
In recent days, both Racing and the player fielded a wave of approaches from clubs trying to change his mind, including Barcelona, Villarreal and Atletico Madrid, as well as sides from the Premier League, Bundesliga and even the Saudi Pro League.
All of it came to nothing against Martínez's firm resolve to join Real Madrid, with the player himself ultimately the one who turned down the rival offers rather than his club making the decision for him.
As in previous deals of this kind, Real Madrid opted against simply triggering Martínez's release clause, set at €3m, instead negotiating directly with Racing to secure the transfer without souring relations - a negotiation Valdebebas had been quietly working on for some time.
The club's interest in the midfielder is not a recent development either: Real Madrid have monitored him for more than a season, and while his emergence in LaLiga has sped up the timeline, the interest itself long predates his top-flight breakthrough.