The Brazilian moves to Estadio do Dragao after failing to firmly establish himself as a regular for Real.
Casemiro made 25 appearances in all competitions last season, but only four of those came as starts.
The 22-year-old failed to find the net for the Spanish capital club last term and will hope to enjoy better individual fortunes with Porto.
Porto could activate an option to make his deal permanent should he impress in the Primeira Liga.
Casemiro is the third La Liga player to join Porto on loan in the transfer window, after Barcelona's Cristian Tello and Atletico Madrid's Oliver Torres to the 27-time Portuguese champion.
Porto has recruited extensively in the close-season, with the likes of Adrian Lopez and Bruno Martins Indi also moving to the club on permanent deals.