The genus Atlantirivulus includes 11 species distributed along coastal river basins and lagoons of eastern and southern Brazil. Most killifishes are threatened with extinction mostly due to their restricted distribution ranges, small population sizes, and habitat loss. Poor knowledge on ecological- and life history information further increases the threats. Atlantirivulus riograndensis (Costa et Lanés, 2009) is a non-annual, small-sized fish endemic in the Laguna dos Patos basin and adjacent coastal areas in southern Brazil. There is no information on the life history and ecology of the species. In this paper, we analyzed population traits of A. riograndensis to support conservation efforts. The fish were captured with a D-shaped hand net in six sampling surveys carried out between 2008 (June, August, and October) and 2009 (March, August, and October) in the type locality of A. riograndensis. The variation of abundance, body size, and condition factor over time, sex ratio, and length–weight relations were analyzed. The estimated abundance was 1.97 specimens per 1 m2. The TL means increased along the year, and males were larger than females. Strong differences in proportions of sexes were detected, with high bias for females. The species showed allometric positive growth (b = 3.3), and the general condition factor (CF) was 0.258 ± 0.003. Atlantirivulus riograndensis inhabits small water bodies and apparently has a small population size. The population is female biased, and males are typically bigger than females. The species has an allometric positive growth. The body size variation is related to individual growth throughout each study year, and probably affected by high mortality of large individuals in dry season. This study presents new original data on basic traits of the natural history of A. riograndensis. The observed results may be used to assist the development of management strategies and conservation of the species and its habitat, mainly in a region where 90% of wetland systems were lost by agriculture, and the remaining areas are at high risk due to the expansion of exotic pine plantations.