City have signed Fluminense forward Kayky for a fee of £9million plus add-ons.
Kayky, 17, has been reported to be of interest to City for some time, with the club beating off competition for his signature.
The right winger has two goals and one assist in nine senior appearances - including one sensational solo goal - and continues City's recent record of signing young Brazilian talent.
Diego Rosa and Yan Couto have also been signed in the last year, with Couto sent out on loan to sister club Girona.
City posted to social media: "We are delighted to confirm we have reached an agreement with Fluminese regarding the future transfer of Kayky. The teenage forward will remain with Fluminese until the end of the Brazilian season."