Castellnou de Bages is a municipality in the province of Barcelona, within the autonomous community of Catalonia, Spain. It lies in the comarca of Bages, in the interior of Catalonia, north of the city of Barcelona.
The municipality covers an area of 29.16 square kilometres (11.26 square miles). Its population in 2021 was 1,431, making it a small and sparsely populated rural municipality.