Zouma becomes new Everton manager Marco Silva's seventh and final summer signing and one of four to come about on the final day of the transfer window.
Deals for Bernard, Yerry Mina and Andre Gomes were struck by the Goodison Park club on Thursday, but Zouma's was subject to a late deal sheet.
Everton were forced to wait until Friday morning to complete and announce Zouma's loan signing.
A former Saint-Etienne youth prodigy, Zouma joined Jose Mourinho's Chelsea in 2014 but fell out of favour under Antonio Conte, only 13 appearances in their title-winning 2016-17 season.
Zouma spent last season on loan at relegated Stoke City, making 34 appearances in their doomed Premier League campaign. The 23-year-old is likely to line up alongside Mina in the centre of Silva's defence.
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