Nephroselmis is a genus of green algae. It has been placed in the family Nephroselmidaceae, although a 2009 study suggests that it should be separated into its own class, Nephroselmidophyceae. One species can be an endosymbiont of Hatena arenicola.

The cell body is right-left flattened. On the ventral side are two heterodynamic unequal flagella, the shorter beats towards the anterior direction while the long trails behind. Nephroselmis have a single cup-shaped chloroplast that contains an eyespot in its anterior-ventral edge below the short flagellum and a pyrenoid with starch plates. Vacuole is located in the left-anterior side, nucleus is located in the right-posterior side. The flagellar root system consists of a rhizoplast and three microtubular roots, one of which is multilayered.

All the cells surface area is covered with layers of unminerallized scales, while the surface of the flagella has hairs as well. The cell body surface is covered by two to four layers of body scales. Flagella are covered with at least two scale layers, the second is characteristically rod-shaped. On top of that Nephroselmis has either a third layer of flagellar scales or flagellar pit scales.

The scales morphology can be an important taxonomic characteristic for differentiating between the species. In the case of Nephroselmis spinosa, that exhibits some distinctive morphological characteristics, one of them is a unique outer scale type that has a spine that is extended for about 1μm, is slightly curved and has a hook shape at the end.

A unique symbiosis occurs between the katablepharid Hatena arenicola and Nephroselmis rotunda. One of the interpretations to the observations of their interaction is that this symbiosis stage is leading to a secondary endosymbiosis, which will produce a completely new life form.

This symbiosis is distinct from other reported cases of ongoing symbiosis in that the symbiont plastid is selectively enlarged up to ten times than the normal size.

There is a possibility that retention of Nephroselmis-derived compartments is a precondition of the cell division process of H. arenicola, due to the fact that cell division was only observed in H. arenicola individuals after they have incorporated the N. rotunda parts into their cell body.

The genus Nephroselmis according to reports and documentations done until 2009 appears to have global worldwide distribution. All species of Nephroselmis are marine inhabitants except for Nephroselmis olivacea which is found strictly in freshwater environments.