Postiga will be on loan to the Greek side until the end of the season, but could sign a permanent deal in the summer.
“I have joined a famous European club who are in the habit of winning trophies,” said Postiga.
“The fact Panathinaikos have a Portuguese coach is a plus for me, but not the only reason I accepted their offer. I will also get the opportunity to challenge for the Greek championship, the cup and the Uefa Cup.”
