磷虾是一种类似虾的海洋无脊椎动物。英语名称krill，来自挪威语，意为「鱼的幼苗」。在英语中还有另一个名字euphausiids，源自其拉丁目名Euphausiacea。

磷虾这种小型的甲壳亚门浮游动物是须鲸、蝠鲼、鲸鲨、锯齿海豹及海豹的食物，也是一些海鸟的主要食物。是接近食物链最底部的关键物种（keystone species），不过仍不算最底部，因为牠们会进食浮游植物（phytoplankton）和一些比牠们更小的浮游动物（zooplankton）。在南冰洋的南极磷虾可制造逾5亿公吨的生物质能，约为人类的两倍，每年有逾一半的南极磷虾被鲸、海豹、企鹅、鱿鱼和鱼吃掉，而没被吃掉的那些则会再继续繁殖下一代，以维持种群总数。大部分的磷虾物种每天会在海面和海底之间垂直迁徙一段很长的路线，因此它们的猎食者也会跟着在夜间上海面，日间则潜至海底。

Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name krill comes from the Norwegian word krill, meaning "small fry of fish", which is also often attributed to species of fish.

Krill are considered an important trophic level connection – near the bottom of the food chain – because they feed on phytoplankton and (to a lesser extent) zooplankton, converting these into a form suitable for many larger animals for whom krill makes up the largest part of their diet. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of around 379,000,000 tonnes, making it among the species with the largest total biomass. Of this, over half is eaten by whales, seals, penguins, squid and fish each year, and is replaced by growth and reproduction. Most krill species display large daily vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.