Thesaurus Polyglottus is a lexicographical work published in 1603 in Frankfurt am Main by a German linguist and historian, Hieronymus Megiser. It is a multilingual dictionary that contains more than 8 thousand articles displayed on almost 1600 pages. The Latin entries are followed by equivalents in various languages, mainly European. In each article the author presents as many equivalents as he was able to collect. Portuguese linguistic material is only represented in around 170 articles, which, however, does not make it any less interesting. The article exposes, at various levels (graphic-phonetic, morphological and semantic), the traits of the Portuguese words included in this work which are mostly botanical and zoological terms. Taking these factors into account, we also try to identify the possible sources of the Portuguese vocabulary, among which works of Conrad Gessner and Amato Lusitano from the 16th century stand out.