Prochloron (from the Greek pro (before) and the Greek chloros (green) ) is a genus of unicellular oxygenic photosynthetic prokaryotes commonly found as an extracellular symbiont on coral reefs, particularly in didemnid ascidians (sea squirts).

However this theory is largely refuted by phylogenetic studies which indicate Prochloron is not on the same line of descent that lead to chloroplast-containing algae and land plants.

[2] Prochloron was discovered in 1975 by Ralph A. Lewin of the Scripps Institution of Oceanography.

Prochloron is one of three known prochlorophytes, cyanobacteria that contain both chlorophyll a and b bound to a special light-harvesting protein.

Repeated unsuccessful attempts to culture Prochloron outside a host make them the only known obligate photosymbionts in the phylum Chordata.