Willem Piso (in Dutch Willem Pies, in Latin Guilielmus Piso, also called Guilherme Piso in Portuguese) (1611 in Leiden - November 28, 1678 in Amsterdam) was a Dutch physician and naturalist who participated as an expedition doctor in Dutch Brazil from 1637 - 1644, sponsored by count Johan Maurits van Nassau-Siegen and the Dutch West India Company. Piso became one of the founders of Tropical medicine.
Piso finished his studies in Caen in 1633 and settled in Amsterdam as a doctor, before leaving for Brazil as a private physician and in the company of the painters Albert Eckhout and Frans Post. There he recommended the consumption of fresh fish, vegetables, and fruits after discovering that soldiers and seamen suffered from physical problems resulting from malnutrition.
Together with Georg Marcgrave, and originally published by Joannes de Laet, Piso wrote the Historia Naturalis Brasiliae (1648), an important early western insight into Brazilian flora and fauna. According to reports in the book, besides studying tropical diseases and indigenous therapies (including the use of Ipecacuanha-root and leaves of the Jaborandi), Piso collected plants and animals in Brazil.