Under extreme environmental conditions such as dehydration and high salinity combined with high light irradiation, higher plants are difficult to survive; however, microalgae are found to thrive under such conditions in which some unknown mechanisms that protect against photooxidative stress must be functioning. A green microalga, Coelastrella astaxanthina Ki-4 (Scenedesmaceae), which was isolated as a novel species of the genus Coelastrella from an asphalt surface in midsummer, was found to produce a large amount of orange water-soluble astaxanthin-binding carotenoprotein, named AstaP, under photooxidative stress conditions. Although lipid astaxanthin, a potent antioxidant known as a natural sunscreen, is known to accumulate in eukaryotic microalgae and confer photoprotection, the use of the water-soluble form of astaxanthin was unknown in eukaryotic plants including microalgae. AstaP (recently named AstaP-orange1) conserved fasciclin domain found in secreted and cell surface proteins and possessed a heat-stable activity that quenches singlet oxygen. Recently, three further AstaPs, AstaP-orange2, AstaP-pink1, and AstaP-pink2, were isolated from the microalga Scenedesmus obtusus Oki-4N, and the AstaPs could be classified into two phylogenetic clades. The homologs of the AstaP protein are widely distributed in other organisms, but their function and role remain unknown. Based on this background, we position AstaPs as proteins that enable astaxanthin to exhibit its high antioxidant activity in aqueous solutions. Along with the results of the functional analysis of AstaPs, we discuss some remaining questions and future perspectives regarding these novel pigments involved in plant photoprotection.