This dissertation provides the most extensive description and analysis yet available for Tuparí, an endangered Tupían language spoken by approximately 350 people in the Brazilian state of Rondônia. Previous work on Tuparí discussed basic phonology and morphology only (Caspar and Rodrigues 1957, Seki 2001, Alves 2004); this dissertation, in contrast, addresses a wide range of grammatical questions with a special focus on the syntactic organization of the Tuparí clause. All the data presented and analyzed here were collected by the author over the course of over eight months of on-site field research in Rondônia. Following the best practices of documentary linguistics, I prioritize naturally-occurring data over elicited examples throughout the dissertation. Much use is made of the texts included in a 2016 literacy workbook edited by myself in collaboration with several indigenous schoolteachers, as well as from a separate text collection now in progress.,Chapters Two, Three and Four describe the morphology of nouns, lexical verbs and auxiliaries, respectively. Apart from negation/privation (Singerman 2018), the nominal domain shows little evidence of elaborate functional structure: adjectival modification is sparse, number marking is optional, and there are no determiners. Lexical verbs, on the other hand, exhibit much more morphological complexity than was stated in previous scholarship; for example, a diverse set of adverbial prefixes demarcates a special morphological slot within the verb for incorporated objects. I further show that Tuparí uses auxiliaries to convey positional, aspectual, and temporal meanings. A striking property of these auxiliaries — as well as lexical verbs that express movement — is number agreement manifested through root-internal suppletion. This suppletion demonstrates that Tuparí grammar actively distinguishes between singular, paucal, and plural arguments, even though NPs and pronouns do not overtly realize this three-way contrast.,Chapter Five examines the syntactic organization of the language. The Tuparí clause consists of three distinct layers of headedness: head-final structure is found from the Verb Phrase up through the Evidential Phrase; head-initial structure obtains in the C domain, instantiated by second position (2P) clause-typing particles; and the Tense Phrase, sandwiched in between the CP and EvidP, exhibits a mixture of head-final and head-initial properties. The category of tense in Tuparí is elaborate and heterogenous: mutually exclusively post-verbal auxiliaries, 2P particles, and predicate-final suffixes collectively express a nuanced system of gradations in the past as well as various present- and future-marking strategies. Once this tense system is described, it becomes possible to make sense of those morphemes called `subject pronouns' in prior descriptions (Alves 2004) and `free pronouns' in comparative research on the Tuparí an branch of Tupían (Galucio and Nogueira 2011). These morphemes occur only with a subset of Tense heads and are positionally attracted to those heads in the linear string. I argue that these `pronouns' are not in fact arguments of the predicate but rather the realization of a functional head located in the inflectional layer of the clause. With this finding in place it becomes possible to demonstrate the existence of (at least) two different kinds of null tense marking operative in the language.,Chapter Six addresses the expression of evidentiality in Tuparí. Tuparí marks an obligatory witnessed/non-witnessed contrast through a bound verbal suffix that agrees in number with the subject. This suffix, -pnẽ/-psira, sits immediately underneath Tense within the clausal spine and participates in a nuanced set of interactions with the 2P clause-typing particles. I argue that -pnẽ/-psira can be used only in contexts where the speaker's commitment to the veracity or accuracy of p is presupposed. This presuppositional analysis correctly predicts the interaction between evidential marking and the 2P clause typers; the behavior of the witnessed/non-witnessed contrast within finite embedded clauses, a structural innovation unique to Tuparí among the Tuparían languages; and the incompatibility between -pnẽ/-psira and the counterfactual conditional suffix -kot'oy. The chapter concludes by addressing the origin of -pnẽ/-psira. I present evidence that this morpheme — which bears no resemblance to the freestanding particles that mark evidentiality in other Tupían languages (Gabas 1999, Galucio 2001, Ferreira 2017) — grammaticalized out of the still-productive resultative -psẽ/-pnẽ/-psira, a suffix which agrees with the subject not only in number but in physical position as well.,The appendix provides a description of language's major phonetic and phonological properties, building upon the study of nasal harmony presented in Singerman (2016).