Arsenal were stunned as Galeno scored in stoppage time to give Porto the advantage from the first leg of their Champions League last-16 tie.
The Brazilian winger took a few touches to set himself before curling a wonderful finish into the net, past the despairing full-stretch dive of David Raya.
Sergio Conceicao’s home side should have been ahead at the break after Galeno hit a shot off the post and somehow put the rebound wide.
That was a huge let-off for Arsenal, but Galeno made no mistake in the fourth added minute at the end of the game.
Nico Gonzalez’s powerful strike had earlier sailed over the bar as the home side tried to open the scoring in the first half.
The Gunners dominated possession but failed to have a single shot on target in the match, with William Saliba and Kai Havertz heading wide from corners.
The second leg will take place at Emirates Stadium on 12 March, when Arsenal will aim to reach the quarter-finals for the first time since 2009-10.
Arsenal had plenty of set-piece opportunities but lacked a precise finish. From one of those situations, a corner from Declan Rice was met on the volley by Leandro Trossard who blazed over the bar.