The former Real Madrid and Manchester City manager holds extensive experience in the Spanish top-flight after successful spells with Malaga and Villarreal and has signed a three-year contract.
Pellegrini succeeds Rubi has in the Betis hot seat after the Spaniard was relieved of his duties just three games after the resumption of LaLiga last month.
West Ham United home kit for 2020/2021 season
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Chelsea's home kit for 2020/2021 season
"Real Betis have appointed a coach of a high level and big international prestige to elevate the performance of the first team in a three-year project," the club said in an official statement.
The 66-year-old won league titles in Ecuador, Argentina and England over his distinguished managerial career than began in his hometown of Chile with Universidad de Chile in 1988.
— Real Betis Balompié (@RealBetis_en) July 9, 2020