Diadema is a municipality in São Paulo state, Brazil. Belonging to the ABC Region of Greater São Paulo, it is distant from São Paulo's central point. Initially part of São Bernardo do Campo, Diadema became a city of its own in 1959. The city has an area of and a population of 386,039, the 14th largest in the state. Entirely urbanised, the annual mean temperature in the city is . Its HDI is 0.790.
Although located in the heart of a traditionally industrial region, its main source of income is the service sector, featuring 77 healthcare installations. Diadema is still home to a butterfly zoo, a botanical garden, an art museum and an observatory.