The Sean Dyche era started with plenty of intent on show, as Elliot Anderson fired over inside the opening minute.
Callum Hudson-Odoi then hit two shots in quick succession that had to be dealt with, as Diogo Costa palmed the first away and Alan Varela deflected the second over the crossbar.
The pressure eventually told when Jan Bednarek handled Douglas Luiz’s cross, conceding a penalty that Morgan Gibbs-White clinically converted to put his side ahead in the 19th minute.
Forest also looked solid defensively, as they got blocks in to deny Samu Aghehowa and Victor Froholdt.
There was still a moment of disappointment for the hosts as Igor Jesus was booked for his aerial challenge on Varela, which means the Brazilian misses their match against Sturm Graz.
Matz Sels was given his first notable test in the 38th minute and showed that he was up to the task, palming away Varela’s fizzed strike from range.
He wasn’t called into action again in the first half as Pablo Rosario prodded his shot wide, before Oleksandr Zinchenko made way due to injury.
Having conceded the penalty in the first half, Bednarek suffered yet more disappointment after the restart as his goal was ruled out due to Samu being offside before his shot was saved.
Francesco Farioli took action with a triple change just before the hour mark, but the visitors’ struggles to break down the Tricky Trees continued.
Gabri Veiga did have Sels scrambling to tip his corner delivery over the crossbar, while Hudson-Odoi had an opportunity moments later but saw his attempt blocked by Bednarek.
The hosts had another chance for a second when Martim Fernandes brought down Nicolò Savona, and Igor Jesus took it by sending his spot-kick down the middle.
Forest continued to stand firm to keep a clean sheet as they got off to the perfect start under Dyche by ending a 10-game winless run across all competitions, while also picking up three vital UEL points.
They also ended Porto’s 12-game unbeaten run in all competitions, as well as abruptly halting the Dragões’ perfect start to the league phase.
Flashscore Man of the Match: Neco Williams (Nottingham Forest)