Now 18, the defender is regarded as a future star for club and country at Real Madrid.
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Martinez has recovered from a serious ACL injury suffered last season and penned a new deal to 2029 on Monday.
He confirmed on social media: "I continue to strengthen my relationship with the best club in the world. The support I've received from the club and the fans in this last period is something I will always carry with me.
"Thanks to my friends, my family and everyone around me for making this possible.
"Hala Madrid."