Argentinian newspaper La Capital uncovered the touching picture, showing the footballer and the stunning mum of their three sons as children nearly two decades before they became man and wife, enjoying themselves in the Atlantic holiday resort of Punta Mogotes.
Messi was playing in a youth football tournament in the nearby city of Balcarce.
Antonela, who was about to turn 10 and was already a close friend of the footballer, had travelled to the area with her parents to see her cousin Lucas Scaglia, who was one of Messi's teammates.
The photo, showing the pair with other children including Lucas and another team-mate called Matias Pecce, was taken in January 1998.
Antonela's parents were among the mums and dads who took the youngsters to Punta Mogotes during a day off from the boys' football tournament but Messi's mum and dad, Jorge and Celia, stayed in their native city of Rosario, an eight-hour drive away.