Edinson Cavani had put Boca ahead in the 23rd minute but the Argentine club had defender Marcos Rojo sent off in the 66th minute. Joaquin Piquerez grabbed the leveller for Palmeiras, only for Boca to triumph in the shoot-out, with Romero making two huge saves.
“I’m 36 years old and I’m at an age when I like to have fun, and for me penalties are fun,” said the former Argentina goalkeeper, who had also helped his team secure penalty shoot-out wins in the previous two rounds.
Boca now have a chance to match Independiente’s record of seven Libertadores titles when they face a Fluminense team who are looking for the first triumph in South America’s top club competition.
“You dream about these kind of days when you are a kid and so to live it is a unique, special experience,” said Boca midfielder Ezequiel Fernandez.
“It’s history but we know how big we are, how great we are but it is still a dream,” he added.
Fluminense beat Internacional 2-1 on Wednesday to book their place in the Nov 4 final, which will be held at their home venue, the Maracana in Rio de Janeiro.
Boca grabbed the lead in the 23rd minute with a goal created by two Uruguayans – Miguel Merentiel did well down the left flank, bustling past Gustavo Gomez and putting in a low ball which veteran striker Cavani slid in from less than a metre out.
Valentin Barco then tested Palmeiras goalkeeper Weverton with a low drive from 23m through a crowded penalty area which the Brazilian got down well to save.
Boca’s back line was solid, frustrating Palmeiras and reducing them to efforts from distance.
The Sao Paulo side made a double change at the break – bringing on the highly-rated 17-year-old Endrick and 20-year-old forward Kevin to inject some pace and energy into their attack.
The move paid off with Endrick, who has an agreement to join Real Madrid when he turns 18 next July, troubling the Boca back line with his sharp movement.
Boca’s Romero was forced into two saves, keeping out efforts from Mayke and then Ze Rafael as the home side piled on the pressure.
But then a moment of madness from Boca’s experienced central defender Rojo in the 66th minute, which saw him being sent off for a second yellow card after a late, lunging challenge on Kevin.
Palmeiras smelt blood and piled forward in search of the leveller and Romero showed great reflexes to parry a firm, glancing header from Rony.
The Brazilians got their reward when Boca cleared a corner out to Joaquin Piquerez, who unleashed a fierce drive from 23m which took a slight deflection, and although Romero got a hand on the shot, it whistled into the net.
Palmeiras pushed for a late winner but Romero denied them again with a fine save from a brilliant Rony overhead kick while at the other end, Cavani could have won it on the break but he slipped as he took a shot from a great position on the left.
With neither an away goals rule nor extra time in the competition, the contest went to penalties, with Cavani missing the first kick for Boca.
But Romero then saved the first two kicks from Palmeiras, diving to keep out efforts from Raphael Veiga and Gustavo Gomez.
There were no more errors, however, and Guillermo Fernandez kept his cool with the final spot kick for Boca to seal the win and the place in the final.