Pascal Gross, Charalampos Kostoulas and Chema Andrés scored as Arsenal suffered their first defeat of the season after seven consecutive wins in all competitions.
Arteta went back to his senior players after making nine changes at Ipswich in midweek. Jurrien Timber returned for his first start since March, while Bruno Guimaraes was handed his first Premier League start for the club alongside Declan Rice. David Raya, Riccardo Calafiori, Martin Odegaard, Bukayo Saka and Kai Havertz were also among those restored to the XI.
Brighton caused problems early for the Gunners, giving them little chance to settle into the game. Raya was called into action to deny Charalampos Kostoulas, while Yasin Ayari also saw an effort blocked from a corner during a lively opening from the hosts.
Arsenal gradually began to see more of the ball, with Calafiori heading an opportunity wide and Saka asking questions down the right, but Brighton took the lead after 31 minutes.
Pascal Gross received the ball outside the Arsenal area and curled a right-footed effort beyond Raya, the shot clipping the post on its way in. Only the second time Arsenal had conceded this season.
There was still time for Arsenal to respond before the break, and their best spell of the half arrived late on. Odegaard had an effort saved as Arteta’s side broke forward and Saka also got into a dangerous position inside the Brighton area.
Arsenal pushed for an equaliser, but instead, Brighton scored again.
Kostoulas collected the ball outside the box during a quick Brighton attack, and from an almost identical position to Gross’ first goal, drove a low effort beyond Raya in the final minute of the half, leaving Arsenal with a two-goal deficit to overturn after the interval.
Arteta initially kept faith with the same side, but Brighton quickly made Arsenal’s task considerably harder.
Chema Andrés scored the third from a corner in the 57th minute, heading home from close range to put the hosts in complete control. Arsenal had conceded only three goals across their opening seven games in all competitions before arriving on the south coast, but matched that total inside an hour at the Amex.
Bruno Guimaraes and Timber were withdrawn soon afterwards as Arteta introduced Mikel Merino and Martin Zubimendi.
Arsenal had chances to cut the deficit. Kai Havertz was sent through with a little over 10 minutes remaining but directed his effort straight at Bart Verbruggen, which the keeper dealt with comfortably. With Arsenal becoming stretched to find a way back into the match, Brighton continued to threaten at the other end rather than simply protect their lead.
Raya denied Matt O’Riley late on, with Malick Yalcouye then also striking the crossbar as Brighton pushed for a fourth. Arsenal were unable to find even a consolation before the final whistle.
The defeat ends a run of seven straight victories to begin the campaign, including four from four in the Premier League, plus wins over Napoli in the Champions League and Ipswich in the Carabao Cup.
The Gunners now head into the international break having dropped their first league points of the season, with their next Premier League fixture coming against Leeds United at the Emirates on October 10, allowing time to regroup before the next round of fixtures.
