Green algae (Phylum Chlorophyta) are the group of organisms from which stoneworts and land plants emerged. Green algae are aquatic (either freshwater or marine), photosynthetic organisms that are either unicellular or colonial. It has also been suggested that certain protists have an endosymbiotic relationship with green algae (i.e. Euglenids), independent of green algae

Green algae are morphologically distinct from red algae due to the suspected loss of phycobiliproteins and the presence of the photosynthetic molecule, chlorophyll b. While chlorophyll a absorbs light from the violet to indigo spectrum and the red spectrum, chlorophyll b absorbs light in the blue spectrum and the orange to red spectrum, allowing these organisms to absorb more available light. Neither chlorophyll a nor chlorophyll b absorb light from the green section of the electromagnetic spectrum, which is why green algae and plants appear green.