Real Madrid have activated a buy-back clause to bring in Casemiro from Portuguese side Porto.
The Brazilian international has seven caps for the national side, but spent the whole of the 2014-15 season on loan at Porto.
Porto reportedly had their own purchase clause, but Madrid has the option to override it with their own clause, which they duly did.
Casemiro joined Real Madrid from Brazilian side Sao Paulo in a £6 million deal in July 2013.
Despite his big reputation he largely failed to live up to expectations in his first spell in the Spanish capital, making just 26 appearances for the first team.
Despite that he has enjoyed a good season on loan with Porto – scoring four times in 41 games and attracting the attention of several top European clubs.
His excellent form at club level has earned him a recall to the international side – and he is part of the Brazil squad for the upcoming Copa America.