The 17 year-old striker's move to Real Madrid was confirmed on Monday.
Castellon announced: "CD Castellón and Real Madrid CF have reached an agreement for the transfer of Fran Santamaría to the Madrid club for next season.
"The player leaves Castellón after a long career at his childhood club. The forward has scored 15 goals in 32 games this season. In his second year in the youth ranks, Fran scored two goals in 15 games for CD Castellón "B" in the Segunda Division, in addition to 13 goals in 17 games in the Honor Division with Juvenil "A". Before joining the reserve team, he was the top scorer in his category.
"The club thanks Fran for her exceptional commitment and dedication over all these years, and wishes her the best of luck in her future professional career."