A day after Germany lit up the World Cup with its clinical 7-1 destruction of host Brazil, the Netherlands and Argentina could not manage a goal between them in 120 minutes before the shootout.
Romero - thought to be a weak link in this Argentine team and not even a starter for his Monaco club most of last season - blocked penalties by Ron Vlaar and Wesley Sneijder. For Argentina, Lionel Messi, Ezequiel Garay, Sergio Aguero and Maxi Rodriguez all converted their spot kicks.
"It's luck, that's the truth. You can dive (the right way) and not make it, like happened to their goalkeeper," Romero said. "I had confidence, thank God things turned out well."
In a matchup of two of football's powerhouse nations, two-time champion Argentina will play three-time winner Germany in Sunday's final at the Maracana Stadium in Rio de Janeiro.
That means an extra bitter end to the tournament for Brazilians, who will have to watch their fiercest rivals play for the world title in their most hallowed stadium against a team that humiliated their nation in the semifinals.
It was the second straight penalty shootout following a 0-0 draw for the Dutch. Against Costa Rica in the quarterfinals, coach Louis van Gaal brought on substitute goalkeeper Tim Krul in the last seconds of extra time to replace Jasper Cillessen and Krul saved two spot kicks.
This time, Van Gaal had used up all three substitutions by the end of extra time and Cillessen had to face the shootout. Van Gaal said he would have made the same keeper swap if he'd had a substitution left.
The young Ajax goalkeeper got a hand to Rodriguez's decisive powerful shot, but could only deflect it into the roof of the net, and then collapsed onto his knees and sank onto his back. Krul walked across the pitch to console him.
Van Gaal said he even had a hand in Romero's heroics, having trained him at AZ Alkmaar in the Dutch league.
"Penalties are always a matter of luck," he said. "And I taught Romero how to stop penalties so that hurts."
Many of Argentina's players stripped off their shirts in the rain at the Itaquerao Stadium and danced in front of their fans.
Argentina reached its fifth final, and its first in 24 years. It won the title in 1978 - beating the Dutch - and in 1986. It lost the championship matches in 1930 and 1990. It played West Germany in both the '86 and '90 finals.
The Netherlands, which has never won the World Cup, was seeking to reach its fourth final.
The tournament's second semifinal had been billed as a showdown between Messi and Arjen Robben, but both star dribblers were subdued. Instead it was midfield controllers Nigel de Jong and Javier Mascherano who stood out as both sides' defenses marked two of the World Cup's biggest stars out of the game.
When Robben finally broke free in stoppage time, Mascherano's perfectly timed sliding tackle blocked his shot at the near post.
De Jong, who recovered from a groin injury to start, lasted just over an hour before being replaced by Feyenoord midfielder Jordy Clasie, who made his World Cup debut.
After scoring 10 goals in three group matches, the Dutch scoring dried up in the knockout rounds. The team managed two late strikes against Mexico but failed to find the net before eliminating Costa Rica in the quarterfinal shootout.
"The issue in a championship like this one is that you score one more goal than your opponent, which we didn't do," Van Gaal said, "We didn't create very much."
Argentina also has found goals hard to come by in Brazil, not winning any of its matches by more than a one-goal margin and recording back-to-back 1-0 wins over Switzerland and Belgium in the knockout stages.
The two sides' attacking impotence was highlighted by a 73rd-minute free kick by Messi from the right corner that sailed over everybody and out of play. Dirk Kuyt followed suit a minute later by sending a long ball off the other end of the pitch.
Argentina tried to finish the match in the second half of extra time, but when the chances came Rodrigo Palacio headed tamely at Cillessen and Maxi Rodriguez mishit a volley.
"Argentina didn't create very many opportunities, if any," Van Gaal said. "So there was a balance in the match."