The cardinal tetra (Paracheirodon axelrodi) is a freshwater fish of the family Characidae of order Characiformes. It is native to the upper Orinoco and Negro Rivers in South America. Growing to about 3 cm (1.2 in) total length, the cardinal tetra has the striking iridescent blue line characteristic of the genus Paracheirodon laterally bisecting the fish, with the body below this line being vivid red in color, hence the name "cardinal tetra". The cardinal tetra's appearance is similar to that of the closely related neon tetra, with which it is often confused; the neon's red coloration extends only about halfway to the nose, and the neon's blue stripe is a less vibrant blue.

The cardinal tetra is a very popular aquarium fish, but is less widespread than the neon tetra because until recently, it was difficult to breed in captivity. However, many breeders are now producing the fish; in most cases, one can determine if the cardinal tetra is bred or wild-caught due to damaged fins on wild-caught specimens. Some ichthyologists believe fishkeepers should continue to support the sustainable cardinal fishery of the Amazon Basin, since thousands of people are employed in the region to capture fish for the aquarium trade. If those fishermen lost their livelihoods catching cardinals and other tropical fish, they might turn their attention to engaging in deforestation.