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.
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.
Lacklustre Arsenal punished by streetwise Porto.
In their first Champions League knockout tie since 2017, Arsenal looked nervy and failed to show the dazzling form that has brought them 21 goals in their opening five Premier League games of the calendar year.
For only the second time on record (since 2003-04) the Gunners did not have a shot on target in a Champions League game, previously failing in such a way at Barcelona in the last 16 in March 2011.
Bukayo Saka was kept quiet by the Porto defence while Havertz put his headed chance off target as clear-cut openings were few and far between.
Mikel Arteta’s side were seemingly holding on for a draw but switched off right at the end and were duly punished by the brilliance of Galeno.
Porto sit third in the Liga Portugal, seven points behind Benfica and Sporting Lisbon, but they were always in control against Arsenal, who have yet to win at the Estadio do Dragao in four attempts, drawing once and now losing on three occasions.
The Gunners have won the first leg of just one of their last 12 two-legged knockout ties in the competition (D2, L9), losing each of the last seven first-leg matches.
Arsenal will need a much-improved performance in London next month if they are to ensure it is not eight successive last-16 eliminations from Europe’s top-tier competition.
