Biological collections are central in understanding and preserving life on Earth. In Brazil, the most representative collections are kept by natural history museums, whose primary focus is in invertebrates, vertebrates and vascular plants. Only a few institutions keep repositories in different kingdoms. The Oswaldo Cruz Foundation (Fiocruz), established in 1900, is a strategic public health institution of the Ministry of Health of Brazil. As such, Fiocruz is responsible for a wide range of activities, from basic research to the development and production of vaccines, drugs, reagents and diagnostic kits. Its biological collections were soon established in the expeditions made by naturalists and physicians seeking integrated knowledge of the fauna, flora and tropical diseases. Since then, they have been part of the institutional policy. In a few decades, those collections were already in the forefront of basic and applied research on tropical parasitic and infectious diseases. Currently, they comprise thirty-three repositories representing part of the Brazilian diversity of bacteria, fungi, protozoa, helminths, arthropods, molluscs and plants of medical and environmental importance. Different methods of long-term preservation are applied for the conservation of this wide range of organisms represented by about 6 million specimens. Herein, we describe this range of collections and discuss their complementary role as repositories of groups not represented in other biological collections in Brazil. These valuable biological materials have been used in public health and medical research, as well as for technological development and innovation in Brazil. Parallel to this specific usage, Fiocruz biological collections have played and continue to play a unique and important role in understanding and conserving part of Brazil’s biodiversity that is currently underrepresented in other biological and natural history collections in Brazil and South America.