Porto have filled the gap left by defender Eliaquim Mangala's departure to Manchester City by signing Ivan Marcano from Rubin Kazan.
Marcano, who spent the 2013-14 season on loan with Olympiakos, has signed a four-year contract at the Estadio do Dragao after Porto paid Rubin an undisclosed fee.
The 27-year-old Spaniard will play at centre-back after France international Mangala was sold to City for a reported 40 million euros on Monday.
He told Porto's official website: "I am very happy and proud to come to a club like Porto, who have history of winning titles and some truly fantastic fans, the best there are in football.
"I know what awaits me, and I'm prepared to help the team win and get back to claiming trophies."
Porto finished third in the Primeira Liga last term and have been reinforcing their squad under new head coach Julen Lopetegui.
The most notable new arrivals are winger Cristian Tello and midfielder Casemiro, from Barcelona and Real Madrid respectively, while Netherlands left-back Bruno Martins Indi and Ghana right-back Daniel Opare have strengthened their defence.
Spanish goalkeeper Andres Fernandez, left-back Jose Angel, midfielder Yacine Brahimi and Atletico Madrid duo Adrian and Oliver Torres have also signed for the 27-time Portuguese champions.
Information from the Press Association was used in this report.