Willem Piso (in Dutch Willem Pies, in Latin Guilielmus Piso, also called in Portuguese Guilherme Piso) (Leiden, 1611 - Amsterdam, November 28, 1678) was a Dutch physician and naturalist who participated as an expedition doctor in Dutch Brazil from 1637 - 1644, sponsored by Earl 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 Piso propagated the consumption of fresh fish, vegetables and fruits, after he discovered the soldiers and seamen suffered from physical problems resulting from dietary deficiencies.
Together with Georg Marcgrave and originally published by Joannes de Laet, he wrote the Historia Naturalis Brasiliae (1648), an important early western insight into Brazilian flora and fauna. According to reports in the book, he collected plants and animals in Brazil, besides studying tropical diseases and indigenous therapies. Ipecacuanha-root and leaves of the Jaborandi.
Not only a minor planet, 11240 Piso, is named after him, but also some Nyctaginaceae, the Pisonia.