Ostreococcus tauri is a unicellular species of marine green alga about 0.8 micrometres (μm) in diameter, the smallest free-living (non-symbiotic) eukaryote yet described.

[6] O. tauri was immediately placed in the class Prasinophyceae based on the presence of characteristic chlorophyll pigments and Chlorophyceae-related carotenoids.

[6] Ostreococcus tauri is the dominant algal species, by cell abundance, in the Thau Lagoon in the south of France.

[12] As early as 1998, O. tauri was identified as "a good candidate for biological models such as cell division and/or genome sequencing studies".

[13] Although no sexual life cycle has been described, the occurrence of cryptic sex is indicated by the expression of core genes for meiosis.