Southampton will be playing in European competition for the first time in 12 seasons after Arsenal's FA Cup victory gave the Saints a Europa League spot.
The Gunners thrashed Aston Villa 4-0 in the final at Wembley on Saturday.
Southampton, who came seventh in the Premier League, qualify as Arsenal had already secured their Champions League place by finishing third in the league.
The Saints lost 2-1 on aggregate to Romanian side Steaua Bucharest in the first round of the Uefa Cup in 2003-04.
Ronald Koeman's side will enter the Europa League at the third qualifying round stage, needing to win two two-legged matches to qualify for the group stages of the competition.
Chelsea, Manchester City, Arsenal and Manchester United will be in the Champions League tournament, while Tottenham, Liverpool and West Ham, who qualify through their position in the Fair Play table, join Southampton in the Europa League.
Southampton's first competitive game of the 2015-16 season will now be on 30 July, before the second leg on 6 August. The Premier League season begins on 8 August.