The 52-year-old former Liverpool and Inter manager takes charge of the Blues until the end of the season following the sacking of Roberto Di Matteo on Wednesday morning
The 52-year-old takes charge of the European champions on an interim basis until the end of the current campaign after spending almost two years out of work following his dismissal at Inter.
A statement on the club's official website read: "The owner and the Board believe that in Benitez we have a manager with significant experience at the highest level of football, who can come in and immediately help deliver our objectives.
"The 52-year-old Spaniard is due to meet the players at the training ground in Cobham tomorrow [Thursday]."
Benitez will take charge of his second Premier League club after a successful six-year spell at Liverpool between 2004 and 2010.
His time on Merseyside led to success in Europe as he won the club's fifth European Cup in his second season in 2005, while he also secured the FA Cup a year later.
Roman Abramovich relieved Di Matteo of his duties on Wednesday morning despite the Italian guiding the club to their maiden Champions League trophy and a seventh FA Cup last season.
Di Matteo signed a two-year deal in the summer, but has seen his position questioned almost continually since with Pep Guardiola thought to wanted by the Russian oligarch to lead the club.
Benitez becomes the ninth Blues manager in nine years in the Abramovich era.