Background Planarian flatworms are best known for their impressive regenerative capacity, yet this trait varies across species. In addition, planarians have other features that share morphology and function with the tissues of many other animals, including an outer mucociliary epithelium that drives planarian locomotion and is very similar to the epithelial linings of the human lung and oviduct. Planarians occupy a broad range of ecological habitats and are known to be sensitive to changes in their environment. Yet, despite their potential to provide valuable insight to many different fields, very few planarian species have been developed as laboratory models for mechanism-based research. Results Here we describe a previously undocumented planarian species, Girardia guanajuatiensis (G.gua). After collecting this species from a freshwater habitat in central Mexico, we characterized it at the morphological, cellular, and molecular level. We show that G.gua shares features with animals in the Girardia genus but also possesses traits that appear unique to this species. Conclusion Species comparisons are a powerful tool for dissecting both genotype to phenotype and ecology and evolution relationships. By thoroughly characterizing a new planarian species, our work facilitates future species comparisons as well as the molecular dissection of specific G.gua traits.