The 19-year-old forward has penned a five-year contract with the English giants.
Moreno, however, will spend the coming season on loan at Deportivo La Coruna.
“City are one of the best clubs in the world," he told the club's official website.
"It will be nice to experience being part of such a big club. I want to make the most of it.
“Pep Guardiola is one of the best managers in the world. I think he likes young players very much and he can improve young players tactically and technically. It will be great to work with him and to be part of a great team.”
Moreno is City's seventh summer signing following the acquisitions of Leroy Sane, Ilkay Gundogan, Nolito, Oleksandr Zinchenko, Gabriel Jesus and Aaron Mooy.