Arsenal’s return to the knockout stages of the UEFA Champions League did not go to plan after a superb goal from Galeno in the fourth minute of second-half stoppage time gave Porto a 1-0 win in the first leg of their last-16 tie.
Galeno curled in a brilliant shot to give the Portuguese team the advantage ahead of the second leg at Emirates Stadium.
After scoring 11 goals in their last two Premier League matches, Arsenal struggled to create chances, failing to register a shot on target for the first time this season.
How the match unfolded
Mikel Arteta named the same starting XI for the third consecutive match as Arsenal looked to continue their fine goalscoring form.
And his side were on top in the early stages as they dominated possession.
But the best chance of the first half fell to Porto. Galeno pounced on a loose ball and smashed it against a post before somehow putting the rebound wide.
William Saliba and Kai Havertz had half-chances, both getting on the end of corners from Bukayo Saka, but neither could test Diogo Costa in goal.
The second half was a very similar story to the first as Arsenal had more of the ball but struggled to create any clear opportunities.
The match seemed to heading for a draw when in the final minute of stoppage time, after Gabriel Martinelli's ambitious pass had been intercepted, Galeno received the ball and produced a superb curling effort from outside the box to beat David Raya.
It was the first time Arsenal have conceded a 90th-minute winner in the competition as well as Porto's latest winner on record.
The second leg will take place on 12 March. Arsenal will hope to return to winning ways in when they take on Newcastle United this Saturday as they seek to close the gap on leaders Liverpool to two points.