The use of molecular methods is altering our understanding of the microbial biosphere and the complexity of the tree of life. Here, we report a newly discovered uncultured plastid-bearing eukaryotic lineage named the rappemonads. Phylogenies using near-complete plastid ribosomal DNA (rDNA) operons demonstrate that this group represents an evolutionarily distinct lineage branching with haptophyte and cryptophyte algae. Environmental DNA sequencing revealed extensive diversity at North Atlantic, North Pacific, and European freshwater sites, suggesting a broad ecophysiology and wide habitat distribution. Quantitative PCR analyses demonstrate that the rappemonads are often rare but can form transient blooms in the Sargasso Sea, where high 16S rRNA gene copies mL-1 were detected in late winter. This pattern is consistent with these microbes being a member of the rare biosphere, whose constituents have been proposed to play important roles under ecosystem change. Fluorescence in situ hybridization revealed that cells from this unique lineage were 6.6 ± 1.2 × 5.7 ± 1.0 μm, larger than numerically dominant open-ocean phytoplankton, and appear to contain two to four plastids. The rappemonads are unique, widespread, putatively photosynthetic algae that are absent from present-day ecosystem models and current versions of the tree of life.