The 33-year-old Argentina international has approved a two-year contract with Chelsea, worth £7.5 million, providing an experienced option amid questions over Robert Sánchez's recent errors.
Martínez helped Aston Villa win the Europa League last season but had slipped behind new signing Zion Suzuki. This permanent transfer strengthens Chelsea's goalkeeping depth alongside Mike Penders, as Villa balances their squad with other potential departures.