Floristic classification of woodlands and shrublands of the Uruguayan sierras: The woodland ecosystems of Uruguay have been only described physiognomically, and the controls of their heterogeneity are still poorly understood. The aim of this paper is to describe the variations in species composition of the woody vegetation of the Eastern and Northeastern hills of Uruguay. In three selected areas under extensive cattle grazing, 122 vegetation samples were collected. We identified five main vegetation units: Myrceugenia euosma and Baccharis dracunculifolia shrublands, Syagrus romanzoffiana woods, Myrsine coriacea woods and Eugenia uniflora woods. Correspondence analysis was carried out to describe floristic gradients. The first axis showed a physiognomic gradient from shrub to wood formations, partly determined by water availability linked to soil depth, a factor related to geomorphology, while the second axis revealed differences among communities related to geology. It is the first floristic classification of the woody vegetation in Uruguay, and it proves that, despite the great structural uniformity, the woody vegetation of these hills is floristically heterogenous. Little importance has been given to the geological and edaphic factors responsible for the woods variation in the past, and no descriptions of the shrublands existed. The classification proposed constitutes an alternative to the physiognomic typology used in Uruguay to describe woody vegetation. It shows the limitations of this typology, offers new categories that allow for comparisons with Southern Brazil communities, and provides useful information for designing regional conservation strategies.