Trumai is a genetic isolate language spoken in Brazil. This grammar describes the main aspects of Trumai, with hopes of contributing to typological studies. In describing and analyzing the linguistic data, I begin with synchrony, but I sometimes utilize internal reconstruction to help understand certain grammatical patterns. Trumai is basically an isolating language. There are few inflectional morphemes and words usually consist of a single morpheme. There are four open classes in Trumai: nouns, verbs, adjectives, and adverbs. Nouns and verbs clearly constitute two different classes; adjectives share some characteristics with nouns and verbs, but constitute a class on their own. The distinction between alienably and inalienably possessed nouns is manifested in several constructions, as is the distinction between humans, animates, and inanimates. As expected, the transitive verb phrase in Trumai contains the O and the V; surprisingly, the intransitive verb phrase contains the S, meaning that the Trumai VP is Absolutive-V. There are four verbs types in the language: Intransitive, Extended Intransitive, Transitive, and Extended Transitive. There are also auxiliaries, which can be subdivided in two sets: Aspect-Mood and Spatial-orientation (with the further subdivision of Body Posture and Directional auxiliaries). Trumai has rich syntax, although word order can change because of pragmatic factors. The case-system shows an Ergative-Absolutive alignment. With regard to grammatical relations, the traditional relations of 'Subject', 'Object', and 'Indirect Object' do not play a central role in Trumai grammar. The case-marking system and the syntax of the language consistently identify three argument types: Absolutive, Ergative, and Dative; each type contains a subset of the traditional relations. Other interesting facts observed in Trumai: (i) when a Transitive verb is causativized, both the causer and the causee are marked as Ergative; (ii) the main strategy for voice manipulation is argument suppression; (iii) there is a construction that could be classified as an instance of "possessor ascension"; (iv) the verb of a complement clause behaves as an inalienably possessed noun (more specifically, as a body part term); (v) there are multiple types of negative clauses in Trumai.