Reiss Nelson scored the only goal of the match as Arsenal opened their campaign with a narrow victory over FC Porto at Borehamwood.
Chuba Akpom came closest to breaking the deadlock for the Gunners before half-time, while Federico Varela and Luis Palahares threatened for the visitors.
But it was Nelson, the PL2 Player of the Month for August, who made the difference, finishing off a sweeping move with a curling 20-yard shot past Diogo Costa on 51 minutes.
The win lifts Arsenal into second place in Group E, level on points with their beaten opponents, who have played a match more.
Reading lie fourth, level with third-placed Bayern Munich, who have yet to play.
Watch Nelson's winner
It ends #AFCU23 1-0 Porto - and what a way for @ReissNelson9 to win it for us 👏 pic.twitter.com/4zxKnXTRNx
— Arsenal FC (@Arsenal) November 17, 2017
Next fixtures
Arsenal: 20 Nov v Swansea City (A) PL2
Porto: 26 Jan v Bayern Munich (A) PL International Cup