What happened: The 33-year-old center back agreed to a contract through the 2028-29 season. His arrival marks another defensive addition under manager Tata Martino as the club retools its lineup.

Why it matters: Securing an experienced international central defender stabilizes the backline for the remainder of the 2026 campaign. Martino has prioritized defensive organization and leadership as Atlanta pushes for postseason positioning during the summer transfer window.

What to watch: Alonso will await international clearance and visa approval before integrating into training sessions and making his match debut.