ESPN journalist Rodra broke the news on Tuesday, confirming that Castellón's contract at Rayo Vallecano expires on 30 June, allowing him to join Madrid without a fee. Bellosillo, also a product of Rayo's youth academy, is expected to complete a similar move.
Castellón is a left winger with pace and technical ability who has caught the eye of Madrid's youth scouting department over the past two seasons.
He will join Madrid's Castilla setup initially, with the potential to be integrated into the senior environment depending on his development trajectory.
The signing is part of Madrid's ongoing strategy of raiding the academies of Spanish clubs for players in the final year of their youth contracts — a model that has previously delivered Marcos Asensio, Álvaro Morata and more recently Gonzalo García through the Castilla pathway.
No official statement has yet been released by Real Madrid, with a formal announcement expected in the coming weeks once the season concludes.