Summary The presence of relict non-photosynthetic plastids in obligate intracellular apicomplexan parasites (e.g. Plasmodium ) has proved puzzling in many ways, but the recent discovery of their photosynthetic relative, Chromera velia , has begun to shed much-needed light on the origin and evolution of these plastids [1,2]. The intense interest that this single species has generated demonstrates how surprisingly little we know about photosynthetic relatives of apicomplexans as a whole. Here, we investigate global plastid diversity and distribution by comprehensively searching existing prokaryotic sequence surveys for eukaryotic plastids. From more than 1.6 million bacterial sequences, we identified 9,799 plastid-derived sequences, most of which were previously mis-labeled as ‘novel bacteria' sequences. 98.8% of these plastid-derived sequences could be assigned to well-defined algal lineages, most often green algae, diatoms, and haptophytes. The exceptions were 121 sequences, all of which were related to apicomplexan parasites, and nearly all of which were derived from coral reef environments. Close relatives of C. velia were rare, but two other clusters were more common and globally distributed, one of which was tightly associated with corals. Overall, all of the major new lineages of algae we discovered were related to apicomplexans, suggesting that apicomplexans represent a large pool of unexplored algal diversity.