The yellow swordtail (Xiphophorus clemenciae) is a species of freshwater fish in the family Poeciliidae.

[2] It is endemic to the upper Coatzacoalcos River basin in southern Mexico.

[2] The yellow swordtail is considered a threatened species by Mexican authorities.

[3][5] The southern mountain swordtail (X. monticolus), which is found further south in the Coatzacoalcos River basin than the yellow swordtail, is the result of a similar event.

[3] The fish is named in honor of Álvarez' wife Clemencia, whose help and advice made it possible for him to devote himself to the pursuit of his scientific research.