Díaz has signed a five-year contract running until 30 June 2031, with Atletico Madrid retaining a buy-back option over the next three transfer windows as well as the remaining half of his rights.
A graduate of Atlético's academy since arriving from Rayo Vallecano in 2018, he steadily progressed through the club's youth ranks before making his senior breakthrough last season, playing four league matches for Diego Simeone's side; his debut came away at Real Oviedo in February, and two months later he started away at Sevilla itself, where Luis García Plaza's side won 2-1 in a result that proved vital in their fight against relegation. Díaz had also been close to agreeing a move to Levante before Sevilla stepped in.
He arrives at a club currently carrying something of a surplus at left-back: alongside Chilean international Gabriel Suazo, the squad also includes Adriá Pedrosa, back from a loan spell, and Marcos "Oso" whose future has been the subject of market interest for weeks without a departure yet being finalised.
Further outgoings at the position are expected, with sporting director José Ignacio Navarro continuing to work on both sides of the ledger as he shapes the squad, with a move for Real Madrid goalkeeper Fran González reportedly the next deal in the pipeline.
Díaz has represented Spain at every youth level since Under-18, and was part of the side that won the Under-19 European Championship in Northern Ireland in the summer of 2024, providing the assist for the extra-time winner in the semi-final against Italy before starting the final against France. A year later he featured at the Under-20 World Cup in Chile under Paco Gallardo, starting every match of the knockout stage until Spain's quarter-final elimination by Colombia.