This is the second Northern Ireland fan to die in a matter of days.
A Northern Ireland fan collapsed and died during their 2-0 win over Ukraine at Euro 2016 on Thursday, the Police Service of Northern Ireland (PSNI) said.
"The man, in his sixties, collapsed in the upper part of the ground and was treated by paramedics but they were unable to revive him," a PSNI statement said.
The man is the second Northern Ireland fan to die during the tournament after Darren Rodgers, from Ballymena, fell eight metres to his death on Monday after landing on his head on a stone beach having scaled a seafront railing in Nice.
The Irish Football Association issued a statement saying it was "deeply saddened" by the news of the latest death.
"The fan passed away as he was watching the game in the stadium. This puts any result of a football match into perspective," said Irish FA chief executive Patrick Nelson. "The thoughts of the board, management, players and staff are with his family and friends at this sad time."
The stadium was filled with Northern Ireland fans who had travelled to France to watch their country's first major finals since the 1986 World Cup.
The supporters had earlier applauded the 24-year-old Rodgers' memory during the 24th minute of Thursday's match. Rodgers was the first supporter to die during the month-long tournament, which has been marred by off-pitch violence between fans since Friday's opening.
Northern Ireland coach Michael O'Neill had said Rodgers' death put his side's 1-0 defeat by Poland in their opening game on Sunday into perspective.