The Netherlands defender will continue his career in Portugal after impressing at the World Cup earlier this summer.
The Netherlands international has signed a four-year deal with the Portuguese giant, which contains a 40 million-euro buyout clause.
Porto was keen to add another defender to its squad because of the expected departure of Eliaquim Mangala, who is on the verge of joining Manchester City, and it has now found its man in Martins Indi.
The 22-year-old came through the ranks of the Feyenoord youth academy and made his official first-team debut at the Rotterdam side in 2010-11.
Martins Indi had since developed into a key player at De Kuip, making over 100 appearances for the Rotterdammers.
The center back — who was born in Portugal — was a part of the Netherlands team that finished third at the World Cup in Brazil this summer, making six appearances.