The Brazilian midfielder has agreed the short-term spell with a view to a later permanent deal also possible.
The 20-year-old will be joining up with the Blancos' 'B' team for the rest of the current campaign.
Casemiro came through the Sao Paolo academy, and has featured for the national side's Under-17 and U20 sides.
Meanwhile, Castilla midfielder Antonio Martinez has joined CD Mirandes on a six month loan, as he did last season.
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