Brazilian Under-20 international defender Danilo Luiz da Silva signed for Porto, the Portuguese champions announced on Thursday.
Porto had already said on July 19 that they had reached agreement with Brazilian club Santos with a transfer fee of 13m euros.
Danilo, who will join the Europa League winners after the Under-20 World Cup in Colombia, has signed a contract that will keep him at the club until the end of June 2016. However, there is a buy-out clause of 50m euros.
The 20-year-old, who can also play in midfield, was a member of the Santos side which won the Copa Libertadores this year.
He is the sixth player to join Porto during the close season following wingers Juan Iturbe, Kelvin de Oliveira and Djalma Campos, goalkeeper Rafael Bracalli and striker Kleber Pinheiro.
Porto meet Champions League winners Barcelona for the European Super Cup on August 26 in Monaco.