The striker came on for home hero Luis Figo after 75 minutes with England winning the quarter-final 1-0 thanks to Michael Owen's third-minute opener.
Just eight minutes later, Helder rose to head home Simao's cross to level the scores and take the game into extra time.
Rui Costa put Portugal ahead in extra time only for Frank Lampard to level it at 2-2 and the game went to a penalty shoot-out.
Helder scored a cheeky dinked penalty in the shoot-out as David Beckham and Darius Vassell missed and Portugal went through 6-5, goalkeeper Ricardo drilling home the clinching spot kick.
Helder Postiga made an impact few English fans will forget in his first taste of action in Euro 2004 last night.