The 19 year-old attacking midfielder is rated the best young talent outside the top two divisions in Spain.
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Rezola was courted by a raft of LaLiga clubs including Athletic, Real Sociedad and Osasuna.
However, Real acted quickly this week and have paid the €150,000 clause in Rezola's contract ahead of the January market.
Rezola will initally be registered with Real Madrid's C and Castilla teams for the second-half of the season. The move will be confirmed on January 1.