The hosts started strongly but could not test David Raya, with Adama Boiro going closest as his goalbound free-kick was blocked. Arsenal started to gain control as the half progressed, though, and Viktor Gyokeres had two chances to score.
The first produced the half’s only shot on target, as he ran in behind before striking with his weaker left foot, but Unai Simon saved comfortably.
The second was a diving header that flashed wide from Jurrien Timber’s cross, yet the Gunners could not convert as the game remained goalless at half-time.
Nevertheless, Arsenal continued to probe in the second half as Gyökeres headed Declan Rice’s free-kick wide at the back post, before Noni Madueke ran in behind and saw his cross-come-shot beaten away by Simon.
It was then Athletic’s turn to attack as Inaki Williams forced Raya into his first save just before the hour mark with an albeit tame effort.
Ultimately, the visitors had the firepower on the bench to swing the game in their favour, and two substitutes combined for the breakthrough.
Leandro Trossard fed Gabriel Martinelli, who ran through on goal and finished under Simon less than a minute after coming on.
The same two players then combined again to seal the points, as Martinelli found Trossard before the Belgium international fired a deflected strike into the back of the net.
It was cruel on Athletic, who have now suffered successive home defeats, while Arsenal stretched an unbeaten record of opening European matches to nine.
They will now look ahead to the weekend’s Premier League fixture with Manchester City as they look to battle on four fronts.
Flashscore Man of the Match: Gabriel Martinelli (Arsenal)
