The two teams can still qualify for the knockout stages in Russia - Iran have three points, Portugal have four - meaning that their encounter in Saransk is winner takes all.
As such, Iran's fans took to the streets on Sunday night, having apparently discovered the Seleccao's team hotel, and set about making all the noise they could.
The group of supporters were chanting, singing, clapping and blowing vuvuzelas as they attempted to disrupt the preparations of Cristiano Ronaldo and Co.
There was also a video clip being shared on social media which purports to show Ronaldo approaching his hotel room window, pleading with the fans to let him sleep.
Iran, meanwhile, beat Morocco in their opening game, before falling to a narrow 1-0 loss to Spain.
Iran manager Carlos Queiroz is looking forward to the encounter , insisting that this game will be his side's sternest test yet.
"If Morocco and Spain were difficult [games], Portugal is even more difficult. We knew this, we didn't come here to play easy teams," he said.
"Not even in their dreams did my players imagine they would one day play against Cristiano, and all of Portugal's other great players so they are very excited.
"We will try to do our best, concentrate and fight from the first minute. We have nothing to lose, they have everything to lose, we have everything to gain.
"We also hope we have that bit of luck that is always necessary."
A remarkable combination of results could see Spain knocked out, and Portugal and Iran both through; a draw between Iran and Portugal and a two-goal defeat for Spain against the already eliminated Morocco would see La Roja fail to progress to the knockout stages.