Photosymbiosis is a symbiotic relationship between two or more organisms, one of which is capable of photosynthesis. Like other forms of symbiosis, photosymbioses can involve the full spectrum of trophic interactions from mutualism through commensalism to parasitism. As in marine benthic environments (e.g., coral reef ecosystems), photosymbiotic associations are frequently encountered in marine pelagic environments and can involve various combinations of microalgae with bacteria, protists, or metazoans. Here, we aim to provide a brief overview of current knowledge on the diversity of the organisms involved in pelagic photosymbioses, their ecological role, and their relevance for the ecosystem. This chapter focuses on mutualistic interactions occurring between photosynthetic protists and bacteria, between two protists and between microalgae and metazoans, as well as on photosymbiotic interactions involving parasitic protists. A section reviewing the most common and recent approaches used to study pelagic photosymbioses and presenting general perspectives in the field concludes the chapter.