Arsenal were stunned as Galeno scored in stoppage time to give Porto the advantage from the first leg of their UEFA Champions League last-16 tie.
The Brazilian winger curled a wonderful finish into the net, past the full-stretch dive of David Raya.
The second leg will take place at Emirates Stadium on March 12, when Arsenal will aim to reach the quarter-finals for the first time since 2009-10.
Also Victor Osimhen salvaged a draw for Napoli in his first club appearance since December as they held Barcelona 1-1 in their Champions League last-16 first leg.
The second leg also takes place in Barcelona on March 12.