The eastern cottontail rabbit (Sylvilagus floridanus; ECR) is highly adaptable to human activities and widely distributed across the Yucatan Peninsula. However, the eastern part of the peninsula, where the Mexican state of Quintana Roo is located, had only marginal records. Herein, we report the collection of one individual and the observation of 7 different individuals of the ECR in northeastern Quintana Roo. These records increase the range of the species by 66 km into the eastern portion of the Yucatan Peninsula. The absence of previous records of the species—even though the mammalian fauna of the region has been surveyed both historically and recently—probably indicates that the species has expanded its range. Further research is required to determine factors that contribute to the species' range expansion as well as its effects on other species.