The Primeira Liga outift have acted swiftly to replace Eliaquim Mangala, who joined Premier League champions Manchester City earlier on Monday.
Marcano spent the second half of last season on loan at Olympiacos, helping the Greek club to lift the Super League title.
He will now play under the stewardship of Julen Lopetegui, having penned terms on a contract that runs until 2018.
"I am happy and proud to be at a club like FC Porto, with a history full of titles and truly amazing fans, the best in football nowadays," Marcano told the club's official website.
"I know what is expected of me and I am ready to help the team get back to winning titles."
Marcano started his career at Racing Santander before moving to Villarreal in July 2009.
However, he failed to nail down a first-team spot and was subsequently shipped out on loan to Getafe and Olympiacos, before joining Rubin in June 2012.
Marcano then returned to Greece in the January transfer window.
