According to the online bookmaker, 2010 winners Spain are now with Brazil to win the competition.
It was not as straight-forward as many expected, with Carlos Queiroz's Iran proving tough opposition who were difficult to break down.
Costa broke Iran's sturdy resistance on 54 minutes, turning on an Iniesta through-ball in the box and steering home, after the ball had ricocheted off Ramin Rezaeian and back onto the Atletico Madrid striker (see below).
It looked like Iran, who beat Morocco in their opening game, may have snatched a point as Saeed Ezatolahi's finish at the back post was ruled out for offside after a VAR review.
Their 1-0 win leaves Spain joint-top of their World Cup group with Portugal on four points, but Iran still have plenty to play for, just a point behind heading into their final match.
In Wednesday's other games, Portugal beat Morocco 1-0 and Uruguay won with the same scoreline v Egypt. If you backed all three teams in a treble with the correct 1-0 scorelines, Sky Bet would have offered odds of 233/1.
Odds correct as of 2050 BST 20/06/18