Rhodeus smithii, sometimes known as the Japanese rosy bitterling, Japanese bitterling, or Nippon baratanago is a temperate freshwater ray-finned fish belonging to the family Acheilognathidae, the bitterlings. It originates in stagnant waters in inland rivers in Japan. It was originally described as Achilognathus smithii by Charles Tate Regan in 1908, and is also referred to as Rhodeus ocellatus smithii in scientific literature.

Named in honor of British traveler, sportsman and naturalist Richard Gordon Smith (1858-1918), who collected specimens in Japan for the British Museum (Natural History).

It is listed as critically endangered in the IUCN Red List. The fish reaches a length up to 6.5 centimetres (2.6 in), and is native to freshwater habitats with a pH of 6.8 to 7.8, a hardness of 20 DH, and a temperature of 10 to 25 °C (50 to 77 °F). When spawning, the females deposit their eggs inside bivalves, where they hatch and the young remain until they can swim.

It lives about three years and rarely exceeds this lifespan.

It was widespread in the west side of Japan (Kyushu and western part of Honshū) before World War II. In 1942, the rosy bitterling was accidentally introduced with grass (Ctenopharyngodon idella) and silver carp (Hypophthalmichthys molitrix) from mainland China.

The two species are morphologically very similar, but several distinguishing characters are seen, such as the number of longitudinal scales, principal rays in the dorsal and anal fins, and shape of eggs. Also, rosy bitterling has a silvery-white area anteriorly (white lines) on the ventral fin, but R. smithii does not. In comparison, the ventral fin of R. smithii is a dark color. Another notable difference is body size. R. smithii does not commonly exceed 60 mm (2.4 in) in length, whereas rosy bitterling males are larger than 80 mm (3.1 in) and females of the species commonly exceed 60 mm (2.4 in).

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