Postiga joined Tottenham a year ago for €9m. However, although he scored Portugal's equaliser against England in their quarter-final victory at UEFA EURO 2004™, he managed just one goal in the English Premiership.
'Keen to return'
Tottenham sporting director Frank Arnesen said: "There is no doubting Hélder's ability but maybe he came to the Premiership too soon. He was keen to return home and, although we rate him highly, we did not like to see him unhappy."
Meanwhile, Mendes, capped twice by Portugal, has signed a four-year contract. The 25-year-old made ten Champions League appearances for Porto last season, including the 90 minutes of the final against AS Monaco FC, having arrived from Vitória SC in summer 2003.
Arnesen added: "It is an excellent deal for us because Pedro gives us more options in midfield and brings with him experience in European competition. He has a well-deserved reputation on the continent and we are delighted to have added him to our squad."
©UEFA.com 1998-2014. All rights reserved.