A 72-year-old Brighton supporter died after becoming unwell during the second half of Saturday's Premier League fixture against Fulham, Brighton confirmed.
The man, who was in the upper tier of the East Stand at the Amex Stadium, received prolonged CPR and defibrillation from medics at the scene but passed away following the conclusion of the match, which ended in a 1-1 draw.
Brighton chief executive Paul Barber said: "This was an incredibly sad end to the match this afternoon, and our deepest sympathies and condolences go out to the family and friends of the gentleman who lost his life.
"While the gentleman concerned is foremost in our minds, we are also aware that it was an emotional and distressing situation for staff and supporters in the direct vicinity.
"Over the coming days we will ensure those affected are properly supported."
Brighton's safety team, with the support of Sussex Police and the Premier League, opted not to stop the game because the majority of fans and both sets of players were unaware of the incident.
It was felt any stoppage in play would have brought further attention to the scene and potentially impacted resuscitation attempts, as well as creating possible complications had the man responded and required rapid transportation to hospital.