The Netherlands international has signed a four-year deal with the Portuguese giants which contains a €40m(£31.7m) buyout clause.
Porto were keen to add another defender to their squad because of the immient departure of Eliaquim Mangala, who is on the verge of joining Manchester City, and they have now found their 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 centre-back - who was born in Portugal - was part of the Netherlands team who finished third at the World Cup in Brazil this summer, making six appearances.