The taxonomy of the western Malagasy stingless bees (Liotrigona spp.) has been complicated by the high degree of morphological similarity between the described species. This group includes one of the smallest bee species of the world, Liotrigona bitika (Brooks & Michener, 1988). However, the description of this species is solely based on size differences, and observation of individuals intermediate in size between Liotrigona bitika and Liotrigona madecassa (Saussure, 1890) have cast doubt on the distinctness of the former taxon. I examined specimens, collected in the dry deciduous forest of Kirindy, representing all three of the described Liotrigona species of western Madagascar using a combined approach of size measures and DNA sequence analysis of the cytochrome oxidase I barcoding region. The analysis revealed all described taxa to be valid and furthermore uncovered the existence of a fourth taxon, described as Liotrigona kinzelbachi sp. n. The sympatric species exhibit non-overlapping body size ranges, indicating a strong influence of body size in niche differentiation of this otherwise morphological highly similar clade.