Spliceosomal introns play a key role in eukaryotic genome evolution and protein diversity. A large Rab GTPase family has been identified in a unicellular eukaryote Trichomonas vaginalis. However, the characteristics of introns in Rab genes of T. vaginalis have not been investigated previously. In this study, we identified a 25-bp spliceosomal intron in the T. vaginalis Rab1a (TvRab1a) gene, the smallest intron in T. vaginalis to be characterized to date. This intron contains a canonical splice site at both 5' (GT) and 3' (AG) ends, and a putative branch-point sequence (TCTAAC) that matches the Trichomonad consensus sequence of ACTAAC except for the first nucleotide. The position and phase of the TvRab1a intron are evolutionarily conserved in Rab1 homologous genes across at least five eukaryotic supergroups, including Opisthokonta, Amoebozoa, Excavata, Chromalveolata, and Plantae. These results strongly suggest that the TvRab1a intron is likely to be an ancient spliceosomal intron, and it can therefore be used as a phylogenetic marker to evaluate particular eukaryotic groupings. Identification and characterization of the TvRabla intron may provide an insight into the evolution of the large Rab repertoire in T. vaginalis.