The 24-year-old - who recently represented his country at the World Cup in Brazil, where he scored his first senior international goal against South Korea - has signed a five-year contract at the Estadio do Dragao.
A statement published on Porto's official website on Wednesday revealed Brahimi's contract features a minimum release clause of 50million euros.
Porto are wasting no time in the summer transfer window as they bid to rebuild heavily in order to make amends for a disappointing third-placed finish in the 2013-14 Primeira Liga table.
Adrian and Oliver Torres have arrived from Atletico Madrid, Cristian Tello and Casemiro have left Barcelona and Real Madrid respectively to move to Portugal, while Holland left-back Bruno Martins Indi and Ghana right-back Daniel Opare have also been added to coach Julen Lopetegui's new-look squad.
Juan Iturbe, Fernando, Andre Castro, Sinan Bolat, Marat Izmailov and Jorge Fucile have all left the club.