The France Under-21 international had been linked with a switch to Stamford Bridge throughout the January transfer window, and the Premier League club confirmed the deal for an undisclosed fee on Friday.
Zouma will remain with Ligue 1 outfit Saint-Etienne on loan until the end of the season, before moving to Chelsea ahead of the 2014-15 campaign.
Chelsea manager Jose Mourinho confirmed last week that he had opened dialogue with Zouma, saying: "Chelsea thinks about the future ... and when we have permission, as we did in this case, we contact the player."
Zouma has made 52 Ligue 1 appearances for Saint-Etienne to date, scoring four goals.
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