Tetraselmis species vary greatly in cell size and shape. Cells can be round, ovoid, elliptical, flattened, compressed, or a combination of these shapes, in which their side lengths can vary in range from 3.5-25 um. Tetraselmis cells have four flagella of equal length, which emerge from a depression near the apex. In most species, the flagella emerge in pairs and project away from each other.

Tetraselmis species have three life stages which includes a flagellated stage, a vegetative non-motile stage, and a cyst stage. The vegetative non-motile stage is the dominant life-stage of most species. The only structural difference between the flagellated and vegetative stage is the number of scale layers in the thecal-wall; the flagellated stage only ever has one layer, while the vegetative state has two or more.

Tetraselmis species have been important for cellular biology research, for studying plankton growth cycles, and they may have future use in biofuel production. For example, the species T. indica is being used to study how asymmetric cell-division during mitosis results in clonal cell heterogeneity. This species forms a clonal colony in which the cells are morphologically and ultrastructurally different, and many cells specialize and develop independent physical properties.