Carlos Justiniano Ribeiro Chagas, or Carlos Chagas (Portuguese: [ˈkaʁluz ʒustʃĩniˈɐ̃nu ʁiˈbejɾu ˈʃaɡɐs]; July 9, 1879 – November 8, 1934), was a Brazilian sanitary physician, scientist, and bacteriologist who worked as a clinician and researcher. He discovered Chagas disease, also called American trypanosomiasis, in 1909, while he was working at the Oswaldo Cruz Institute in Rio de Janeiro.

Chagas's work holds a unique place in the history of medicine. Working in primitive conditions, Chagas described in detail a previously-unknown infectious disease, its pathogen, vector (Triatominae), host, clinical manifestations, and epidemiology. Chagas was also the first to discover and illustrate the parasitic fungal genus Pneumocystis, which later became infamous for being linked to pneumocystis pneumonia in AIDS patients.