The 17-year-old attacking midfielder will cost Madrid 45 million euros ($52 million) according to Spanish media, while Argentine reports suggest the price may be as high as 60 million euros.

Mastantuono will play in the Club World Cup in the United States this summer with River before moving to the Spanish capital.

"Mastantuono will be a player at our club for the next six seasons, from 14 August 2025 to 30 June 2031," said Los Blancos in a statement.

"At 17, he is also the youngest player to play an official match for the Argentina national team in its history."

The midfielder came through the River academy and made his first team debut for the club in January 2024.

He is Real Madrid's third signing this summer behind Trent Alexander-Arnold from Liverpool and Dean Huijsen from Bournemouth.

Madrid also appointed Xabi Alonso as their new coach to replace Carlo Ancelotti, after failing to win a major trophy this season.