The yellow tang (Zebrasoma flavescens), also known as the lemon sailfin, yellow sailfin tang or somber surgeonfish, is a species of marine ray-finned fish belonging to the family Acanthuridae which includes the surgeonfishes, unicornfishes and tangs. Bright yellow in color, it is one of the most popular marine aquarium fish, though in its natural state, it lives in reefs. The yellow tang spawns around a full moon, eats algae, and has a white barb, located just before the tail fin, to protect itself.

The yellow tang was first formally described by English naturalist Edward Turner Bennett as Acanthurus flavescens in 1828 from a collection in the Hawaiian Islands. Zebrasoma refers to the body and the zebra-like stripes or bars on the body of the type species the genus, the sailfin tang (Z. velifer). Its species name is the Latin adjective flavescens, which refers to the tang's yellow color.

Within the genus Zebrasoma the yellow tang forms a species pair with the scopas tang (Z. scopas). The genera Zebrasoma is one of two genera that make up the tribe Zebrasomini within the subfamily Acanthurinae of the family Acanthuridae.

Based on the gene Cytochrome C-oxidase 1 (CO1), a group of researchers was able to reconstruct the phylogenetic tree of the genus Zebrasoma with mitochondrial barcoding sequences.

Adult fish can grow to 20 centimetres (7.9 in) in length, and 1–2 centimetres (0.39–0.79 in) in thickness. Adult males tend to be larger than females. Yellow tangs are bright yellow in color. At night, the yellow coloring fades slightly, and a prominent brownish patch develops in the middle with a horizontal white band. They rapidly regain their bright yellow color during daylight. They can be aggressive, are prone to marine ich, and may damage coral within a reef tank. Male and female yellow tang look very similar. When mating, however, males change color and have a "shimmering" behavior which makes them identifiable. The yellow tang has 5 dorsal spines along with 23–26 dorsal soft rays. The yellow tang also has 3 anal spines as well as 19–22 anal soft rays. There is a white spine on its caudal peduncle that it can use for defense. Its snout is moderately protruding. Its mouth is small with spatulate teeth that are place classed relatively close together inside of the yellow tang's mouth. In juveniles, there are 12 upper and 14 lower teeth. In adults, there are 18 upper and 22 lower teeth.

The yellow tang is a marine fish that lives in reefs and is typically found by itself or in small groups/schools. It is mainly herbivorous and eats filamentous algae.

Spawning happens throughout the year, and it peaks once, normally occurring around the time the moon is full, so this suggests there is some sort of lunar periodicity to the spawning behavior. Spawning happens in pairs or groups, and fertilization is external. Eggs are left in open water and yellow tang are substratum egg scatterers. Yellow tang do not guard their eggs, and once the eggs hatch, the juveniles receive no parental care.

In the wild, yellow tangs feed on benthic turf algae and other marine plant material. In captivity, they are commonly fed meat/fish-based aquarium food, but the long-term health effects of this diet are questionable. However, most experts in the marine aquarium industry express little scepticism that such a well rounded and balanced diet including plant and animal material would be in any way detrimental to mostly herbivorous fishes like tangs since they still need on occasion, complex amino acids and nutrients that only ocean animals can provide. In the wild, yellow tangs provide cleaner services to marine turtles, by removing algal growth from their shells.