Shane Long has scored the only goal of the afternoon at St Mary's as Southampton edged out Athletic Bilbao in their final pre-season fixture.
The Republic of Ireland international raced onto a long ball by Steven Davis before applying a cool finish in the 43rd minute to secure victory for Claude Puel's side.
The Saints looked in control in the opening stages of the game and almost took the lead moments earlier as Nathan Redmond's strike was deflected against the upright.
Southampton continued to dominate in the second half and looked dangerous from crosses, with substitute Charlie Austin and defender Virgil van Dijk both coming close to extending the lead with headers.
Puel's side, who went the entire pre-season without defeat, take on Watford in the opening match of their Premier League campaign next weekend.