A floristic survey was carried out in Yungas forests of the San José de Chijra river basin (Jujuy, Argentina). This is the first comprehensive study of vascular plants made in the area, a region relatively unknown in terms of vegetation. We present the results of this field survey; 172 species, 127 genera and 51 plant families are documented. Of these, 149 are native and 17 are endemic. Of the latter, three are endemic to Argentina, 11 to northestern Argentina, and three to Jujuy province. Families with the greatest number of species were Asteraceae (50), Solanaceae (13), Fabaceae (12), and Poaceae (8). According to IUCN criteria, two species are classified as threatened. Our data substantially increase our knowledge of the diversity of the regional flora and will provide an important contribution in developing conservation policy.