Prior to 1800, exploring expeditions were usually of a commercial and political nature, undertaken to colonize or expand relations with new countries and territories. Among those to be explored as the 19th century progressed was Brazil, the largest nation in South America. This account is a brief historical overview of some 19th and early 20th century expeditions to Brazil, focusing on those that included investigations of hydrozoans (phylum Cnidaria). Among the more important of these expeditions were l'Expédition de La Coquille (France), the Voyage of the Beagle (Great Britain), the United States Exploring Expedition, l'Expédition HSWMS Eugenie (Sweden), the Thayer Expedition (United States), the Challenger Expedition (Great Britain), the Albatross Expedition 1887–1888 (United States), the Scottish National (Scotia) Antarctic Expedition, the British Antarctic Expedition, and the Hartt Expedition (United States). Maps of routes taken and names of expedition commanders and scientists are provided in the supplemental material online. Also included is a list of hydrozoan species collected in Brazilian waters by the expeditions (with original and current names of taxa), together with dates and exact locations of collection, station numbers with coordinates and depths (when available), and the museums where material is presently housed, with respective catalog numbers and sources of information. Citations of sources of updated species names are also provided. Figures (diagrams and old plates prepared by expedition illustrators) are added.