Midfielder Fernando is the latest Brazilian to switch allegiance after being granted Portuguese nationality.
The 26-year-old has been a mainstay in Porto's midfield since joining the Primeira Liga club from Brazil's Vila Nova in 2007, reports Xinhua.
It is understood the decision to apply for Portuguese nationality came at the behest of Portugal's national coach Paulo Bento, who wants Fernando in his 2014 World Cup squad.
"Finally I've got it," Fernando said on Twitter. "I am proud to say I am officially Portuguese."
Porto manager Paulo Fonseca backed Fernando to make Bento's final World Cup list.
"Maybe I am biased but Fernando is the best footballer in his position in Portugal so of course he would have a place in the national team," Fonseca said.
Fonseca joins a long list of Brazilian-born players to switch nationality.
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Recent examples include Deco (Portugal), Marcos Senna (Spain), Pepe (Portugal) and Diego Costa (Spain).
Fernando's contract with Porto expires in June and he has been linked with a move to Inter Milan.