The 19-year-old revealed earlier this month he had been contacted over the phone by Jose Mourinho, and it is believed that the Portuguese persuaded the youngster to join the Blues
The 19-year-old revealed earlier this month that he had received a phone call from Jose Mourinho, and it is believed that conversation persuaded the youngster to pick the Blues over rival suitor Manchester City.
Saint-Etienne vice-president Dominique Rocheteau later insisted the club would not part with Zouma in January and it is understood that Chelsea will allow him to remain in France until the summer.
Zouma is regarded as one of the best young center backs in Europe, though he is currently serving a 10-match suspension for breaking Sochaux midfielder Thomas Guerbert's leg in November.
Since making his professional debut for Saint-Etienne at the age of 16 Zouma has played over 50 games for the Ligue 1 club, while also representing France at the youth level.
The deal represents another important step in Mourinho's attempts to remodel the Chelsea squad, following the January signings of Nemanja Matic and Mohamed Salah and sales of Juan Mata and Kevin De Bruyne.