This study describes reconstruction of two highly unusual archaeal genomes by de novo metagenomic assembly of multiple, deeply sequenced libraries from surface waters of Lake Tyrrell (LT), a hypersaline lake in NW Victoria, Australia. Lineage-specific probes were designed using the assembled genomes to visualize these novel archaea, which were highly abundant in the 0.1-0.8 μm size fraction of lake water samples. Gene content and inferred metabolic capabilities were highly dissimilar to all previously identified hypersaline microbial species. Distinctive characteristics included unique amino acid composition, absence of Gvp gas vesicle proteins, atypical archaeal metabolic pathways and unusually small cell size (approximately 0.6 μm diameter). Multi-locus phylogenetic analyses demonstrated that these organisms belong to a new major euryarchaeal lineage, distantly related to halophilic archaea of class Halobacteria. Consistent with these findings, we propose creation of a new archaeal class, provisionally named 'Nanohaloarchaea'. In addition to their high abundance in LT surface waters, we report the prevalence of Nanohaloarchaea in other hypersaline environments worldwide. The simultaneous discovery and genome sequencing of a novel yet ubiquitous lineage of uncultivated microorganisms demonstrates that even historically well-characterized environments can reveal unexpected diversity when analyzed by metagenomics, and advances our understanding of the ecology of hypersaline environments and the evolutionary history of the archaea.