Trichuris serrata, a whipworm of cats, can cause inflammation in the cecum and upper portion of the large intestine. It is unknown if the virulence and pathology of T. serrata differ from Trichuris campanula, the other species in cats. Distinguishing the species based on egg size is challenging. In addition, Trichuris eggs can be difficult to distinguish from Capillaria spp. This paper presents the first molecular description of T. serrata. The 18S ribosomal RNA (rRNA) gene was sequenced from male adult worms sourced from two unrelated cats on St. Kitts. Based on the analysis of 651 base pairs, T. serrata was found to be different than any other Trichuris species for which published sequencing of the 18S rRNA gene is available. A dendrogram was developed using Molecular Evolutionary Genetics Analysis version 6.0, and evolutionary history was inferred using the minimum evolution method. T. serrata was found to be most closely related to Trichuris vulpis, the Trichuris of dogs. Further development of the methodology could enable distinguishing T. serrata, T. campanula, and Capillaria spp. infections in cats and aid in diagnosis.