Syndiniales are a parasitic order within the eukaryotic lineage Dinophyceae (Alveolata). Here, we analysed the taxonomy of this group using 43655 18S rRNA gene sequences obtained either from environmental data sets or cultures, including 6874 environmental sequences from this study derived from Atlantic and Mediterranean waters. A total of 5571 out of the 43655 sequences analysed fell within the Dinophyceae. Both bayesian and maximum likelihood phylogenies placed Syndiniales in five main groups (I–V), as a monophyletic lineage at the base of ‘core’ dinoflagellates (all Dinophyceae except Syndiniales), although the latter placement was not bootstrap supported. Thus, the two uncultured novel marine alveolate groups I and II, which have been highlighted previously, are confirmed to belong to the Syndiniales. These groups were the most diverse and highly represented in environmental studies. Within each, 8 and 44 clades were identified respectively. Co-evolutionary trends between parasitic Syndiniales and their putative hosts were not clear, suggesting they may be relatively ‘general’ parasitoids. Based on the overall distribution patterns of the Syndiniales-affiliated sequences, we propose that Syndiniales are exclusively marine. Interestingly, sequences belonging to groups II, III and V were largely retrieved from the photic zone, while Group I dominated samples from anoxic and suboxic ecosystems. Nevertheless, both groups I and II contained specific clades preferentially, or exclusively, retrieved from these latter ecosystems. Given the broad distribution of Syndiniales, our work indicates that parasitism may be a major force in ocean food webs, a force that is neglected in current conceptualizations of the marine carbon cycle.