Profundulus chimalapensis, a new species of cyprinodontid (Cyrpinodontiformes: Profundulidae) from the Coatzacoalcos River, Mexico. Introduction: The Profundulidae family is a representative of the neotropical ichthyofauna of the order Cyprinodontiformes, considered endemic to the Mesoamerican region. It is represented by the genera Tlaloc and Profundulus. Objective: This paper describes a new species, Profundulus chimalapensis sp. nov., endemic to Mexico. Methods: Specimens were collected in the tributaries of the Upper Basin of the Coatzacoalcos River, which were deposited in reference collections. The corresponding body and meristic measurements were obtained, likewise a sample was diafanized and stained for the analysis of its skeletal morphology. Results: The new species differs from its congeners by having the following characters: 13 dorsal fin rays, 16 anal and 15 pectoral rays; dark colored band on the sides; anal fin with a clear distal margin; anterior and posterior margin of the alveolar process of the concave premaxilla; short and wide epiotic processes. Conclusion: A new species of Profundulus with distribution restricted to the Atlantic side of the Isthmus of Tehuantepec is described.