Abstract DNA in cells is associated with proteins that constrain its structure and affect DNA-templated processes including transcription and replication. HU and histones are the main constituents of chromatin in bacteria and eukaryotes, respectively, with few exceptions. Archaea, in contrast, have diverse repertoires of nucleoid-associated proteins (NAPs). To analyse the evolutionary and ecological drivers of this diversity, we combined a phylogenomic survey of known and predicted NAPs with quantitative proteomic data. We identify the Diaforarchaea as a hotbed of NAP gain and loss, and experimentally validate candidate NAPs in two members of this clade, Thermoplasma volcanium and Methanomassiliicoccus luminyensis . Proteomic analysis across a diverse sample of 19 archaea revealed that NAP investment varies from <0.03% to >5% of total protein. This variation is predicted by growth temperature. We propose that high levels of chromatinization have evolved as a mechanism to prevent uncontrolled helix denaturation at higher temperatures, with implications for the origin of chromatin in both archaea and eukaryotes.