The Tiriyo language has approximately 2,000 speakers (whose autodenomination is tareno [tare:nc], the term tiriyo or trio being of uncertain origin) who live in lowland South America, on both sides of the border between Brazil and Surinam. Like most other languages of the Cariban family, Tiriyo is chronically underdescribed. In the 117 years since Crevaux's first word list came out, very little has been written on the language: a few articles on specific points of phonology or grammar, two small tentative dictionaries, and two longer but incomplete sketches. This dissertation is intended as an effort to improve this situation by offering a more detailed description of the Tiriyo language based on extensive field work. It has a traditional format: after an introductory chapter on the Tiriyo people and previous research on the language, it begins with a description of the segmental and suprasegmental phonology, continuing on to the definition of word classes and the description of their morphology and arriving at the syntax, using what could be broadly defined as a functional-typological approach. A certain number of diachronic remarks and hypotheses are added when deemed appropriate; however, the synchronic descriptive goal is always the primary concern. After the basic description, a further chapter examines the lexicon, describing some formal regularities and also exploring its semantics via a closer look at some selected semantic fields. The appendices contain a collection of texts and a preliminary dictionary with grammatical information on every morpheme.