The Harderian gland (HG) of the red-sided garter snake, Thamnophis sirtalis parietalis, is a secretory structure that plays a role in the vomeronasal system by solubilizing semiochemicals such as prey chemoattractants and the female sexual attractiveness pheromone. The HG produces secretions containing pheromone-binding proteins that form a complex with pheromone molecules making them soluble, allowing for vomeronasal detection. Detection of prey chemoattractants and pheromones is essential for both feeding and reproduction. Feeding occurs only in summer and mating only in spring. Because the semiochemicals and the pheromone-binding proteins are likely different in each season, changes in the gross anatomy and histology of the HG were expected to be seen throughout the year. We found this to be true as glands in the summer had larger masses, greater cell heights, and larger lumen diameters than in other seasons. Sexual dimorphism was observed in the histology and morphology of the gland as males had significantly larger HGs, greater cell heights, and larger lumen diameters than those of females. Finally, sexual dimorphism was observed in the greater cell height of males than females in the spring, likely because males respond to and solubilize the sex pheromone in spring, while females do not.