The visitors actually started the first half on the front foot, recording more possession than their opponents, as Evanilson in particular was causing problems.
The striker turned his defender before firing towards the goal, forcing David Raya to palm away to safety.
Before that, Bukayo Saka forced Diogo Costa into a save after cutting inside, but Arsenal’s breakthrough came on the other flank through Leandro Trossard.
The Belgian neatly tucked his third UCL goal of the season into the bottom corner after a perfectly weighted pass from Martin Odegaard.
Though somewhat less imposing after half time, Arsenal had the ball in the net again when Odegaard beautifully chipped Costa, only for it to be ruled out for a foul by Kai Havertz on Pepe beforehand.
Moments later, Francisco Conceicao forced Raya into a decent save on the counterattack, before Jakub Kiwior produced a brilliant block on the rebound.
As the clock ticked closer to full time, Mikel Arteta threw on Gabriel Jesus for Jorginho, clearly showing his intentions of putting the tie to bed in normal time.
The Brazilian was immediately involved, forcing a smart save from Costa from close range, but 1-0 was how the regular 90’ ended.
There was not much goalmouth action in the first half of extra time, with Porto substitute Mehdi Taremi recording the only notable shot, firing wide of Raya’s post.
The second half was just as cagey, as Saka’s effort was blocked in a wonderful Arsenal passing move before Galeno’s long-range strike deflected into Raya’s arms.
Inevitably, the game finished all square on aggregate and would be decided by penalties.
Arsenal had scored all 13 penalties they had taken this season before tonight, including four in their Community Shield triumph, and Odegaard stepped up first.
The captain led by example, sending Costa the wrong way, but Pepe duly responded for Porto.
And Havertz netted too, before Wendell’s penalty was tipped onto the post by Raya to send the Emirates Stadium into raptures.
Saka then scored to give Arsenal a two-goal advantage, before Marko Grujic made it 3-2. Declan Rice converted to make it ‘match point’, and Galeno stepped up knowing he had to score.
Raya saved again after guessing correctly with a feline dive to his left, sending the home crowd wild as the Gunners won their first competitive penalty shootout at the Emirates, after heartbreak against Sporting in the UEFA Europa League last season.
Flashscore Man of the Match: Martin Odegaard (Arsenal)
