The 44-year-old Spaniard had been out of work since departing Real Madrid in January after less than eight months in charge at the Bernabeu. Before that, he led Bayer Leverkusen to the Bundesliga title without losing a game across the entire 2023-24 season, also winning the German Cup in what was his first full campaign as a senior club manager.
Chelsea sacked Liam Rosenior last month and moved quickly to identify a successor. The club held talks with several candidates, including Bournemouth's Andoni Iraola, Fulham's Marco Silva and Crystal Palace's Oliver Glasner, before settling on Alonso as their preferred choice.
Talks had been ongoing for a number of weeks, with Chelsea opting to allow the FA Cup final against Manchester City to pass before making any announcement public. Alonso is understood to have been receptive to the role subject to receiving assurances on the club's footballing direction.
His connection to English football runs deep. The former midfielder spent five seasons as a player at Liverpool, and his name had been raised in connection with the Anfield vacancy as pressure mounted on Arne Slot following a difficult season. Liverpool are reported to be standing by Slot for next season, leaving the path clear for Alonso to take the Chelsea role.
Chelsea fielded their youngest FA Cup final starting eleven since Nottingham Forest in 1991 in Saturday's 1-0 defeat to City at Wembley. Interim boss Calum McFarlane had stabilised the club in recent weeks, earning praise for the team's performances against Liverpool and City despite failing to win either match.
Alonso inherits a squad with significant young talent and a clear direction. His first task will be preparing for what Chelsea hope will be a more consistent title challenge next season.