Jeremy Sarmiento's goal on 21 minutes was the difference in a training exercise scheduled to give players not involved in Saturday afternoon's friendly important minutes.
Tyler Dibling and Kuryu Matsuki both feature after missing last week's training camp in Girona and have only been cleared to play 60 minutes.
Japan youth international Matsuki and Ross Stewart went close for Saints in the opening 20 minutes of a competitive contest.
Brighton then took the lead when Sarmiento, who scored an agonising late goal against Saints for Ipswich in 2023/24, thumped a strike into the top corner.
Some of these players will be listed as subs but the plan is for players to get a full 90 minutes against Brighton. #SaintsFC https://t.co/V5WoZtNEje
— George Rees-Julian (@rees_julian) August 2, 2025
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Saints pushed for an equaliser after half time, but Brighton held them off and claimed the 1-0 win.
The clubs will face each other at 3:30 today in what will be the final pre-season test for Will Still’s side ahead of the Championship season starting.
Southampton plan to give players a full 90 minutes against Premier League opposition, one week removed from hosting Wrexham on the opening weekend.
Taylor Harwood-Bellis, who has been back in training, did not feature, while midfielder Flynn Downes could partner Shea Charles this afternoon.
Southampton XI: McCarthy, Sugawara, Kayi Sanda, Wood, Edwards, Welington, Matsuki (Akachukwu, 47'), Bragg, Dibling, Robinson, Stewart.
Brighton XI: McGill, Veltman, Coppola, Simmonds, Kadioglu (Slater, 47'), Milner, Yalcouyé, Buonanotte, Gruda, Sarmiento, Welbeck.