The dinoflagellate microalga Symbiodinium sustains coral reefs, one of the most diverse ecosystems of the biosphere, through mutualistic endosymbioses with a wide diversity of benthic hosts [1]. Despite its ecological and economic importance, the presence of Symbiodinium in open oceanic waters remains unknown, which represents a significant knowledge gap to fully understand the eco-evolutionary trajectory and resilience of endangered Symbiodinium-based symbioses. Here, we document the existence of Symbiodinium (i.e., now the family Symbiodiniaceae [2]) in tropical- and temperate-surface oceans using DNA and RNA metabarcoding of size-fractionated plankton samples collected at 109 stations across the globe. Symbiodinium from clades A and C were, by far, the most prevalent and widely distributed lineages (representing 0.1% of phytoplankton reads), while other lineages (clades B, D, E, F, and G) were present but rare. Concurrent metatranscriptomics analyses using the Tara Oceans gene catalog [3] revealed that Symbiodinium clades A and C were transcriptionally active in the open ocean and expressed core metabolic pathways (e.g., photosynthesis, carbon fixation, glycolysis, and ammonium uptake). Metabarcodes and expressed genes of clades A and C were detected in small and large plankton size fractions, suggesting the existence of a free-living population and a symbiotic lifestyle within planktonic hosts, respectively. However, high-resolution genetic markers and microscopy are required to confirm the life history of oceanic Symbiodinium. Overall, the previously unknown, metabolically active presence of Symbiodinium in oceanic waters opens up new avenues for investigating the potential of this oceanic reservoir to repopulate coral reefs following stress-induced bleaching.