The Spanish international rarely appears on the scoresheet and was not as effective as usual against Milan, but he has an uncanny knack of grabbing crucial goals on big occasions.
Two penalties from Lionel Messi had put Barca 2-1 up at the Nou Camp before the break, with Milan only needing to score once more to go through on the away goals rule after last week's 0-0 draw at the San Siro.
In the 53rd minute, Messi raced across the edge of the area and had his shot blocked by a defender.
The ball broke free to Iniesta, who showed great composure to wait until the keeper had committed himself before dinking a shot over him to score.
It was only Iniesta's seventh goal in all competitions this season, but the two-goal cushion allowed Barca to ease back and defend their lead from a position of strength with their trademark dominance of possession.
"I think Andres has more goals in him than the statistics show, but he does so much more than just score goals," Barca coach Pep Guardiola said after the holders had completed the win. "He is a fantastic player."
The 27-year-old Iniesta is one of the trio of outstanding midfielders at the heart of Guardiola's team, alongside fellow youth-team graduates Xavi and Sergio Busquets.
More noted for his intricate dribbling, lightning-quick feet and clever link up-play, the waif-like Iniesta has weighed in with some famous goals in his career.
He struck the dramatic late semi-final equaliser from outside the area at Stamford Bridge that guided Barca past Chelsea and into the 2009 Champions League final in Rome, where they went on to beat Manchester United.
In 2010, he grabbed the most important goal in the history of the Spanish national team when he struck the added-time winner against Netherlands in the World Cup final in South Africa.
The World Cup winner has made Iniesta hugely popular across Spain, where he is regularly applauded by opposition fans, but the shy and retiring playmaker is more than happy to let others take the limelight at Barca.
Messi is on course for a fourth consecutive Champions League top scorer award with 14 goals in Europe so far, and he has racked up 58 in all competitions, but Iniesta may still be required to lend a helping hand again before the end of the campaign.
