Pseudomonadati is a kingdom containing approximately one-third of prokaryote species, mostly gram-negative bacteria and their relatives. It is the closest relative of an even larger kingdom of Bacteria, the Bacillati, which are mostly gram-positive bacteria.

The synonymous name "Hydrobacteria" (hydro = "water") refers to the moist environment inferred for the common ancestor of those species. In contrast, species of Bacillati possess adaptations for life on land. Since 2024, the only validly published name for this group is kingdom Pseudomonadati (there used to be none, because no levels above phylum could exist in earlier versions of the Prokaryotic Code).

"Gracilicutes," which was described in 1978 by Gibbons and Murray, is sometimes used in place of Pseudomonadati. However, "Gracilicutes" included Cyanobacteria (a member of Bacillati) and was not constructed under the now generally accepted three-domain system. More recently, a redefinition of "Gracilicutes" was proposed but it did not include a molecular phylogeny or statistical analyses. Also, it did not follow the three-domain system, claiming instead that the lineage of eukaryotes + Archaea is nested within Bacteria as a close relative of Actinomycetota, a tree not supported in any molecular phylogeny.

Pseudomonadati and Bacillati were inferred to have diverged approximately 3 billion years ago, suggesting that land (continents) had been colonized by prokaryotes at that time.

They include these superphyla and phyla: Acidobacteriota, Aquificota, Bdellovibrionota, Campylobacterota, Deferribacterota, Dependentiae, Desulfobacterota, Desulfuromonadota, Elusimicrobiota, FCB superphylum, Myxococcota, Nitrospirota, Proteobacteria, PVC superphylum, and Spirochaetota.

Some unrooted molecular phylogenetic analyses have not supported this dichotomy of Bacillati and Pseudomonadati, but the most recent genomic analyses, including those that have focused on rooting the tree, have found these two groups to be monophyletic.

Together, Pseudomonadati and Bacillati form a large clade containing 97% of prokaryotes and 99% of all species of Bacteria known by 2009, and placed by Battistuzzi and Hedges in the proposed taxon Selabacteria, in allusion to their phototrophic abilities (Greek σέλας = light). Currently, the bacterial phyla that are outside of Pseudomonadati + Bacillati, and thus justifying the taxon Selabacteria, are debated and may or may not include Fusobacteria.

The definition of two major divisions within the domain Bacteria, Pseudomonadati, and Bacillati, has come largely from rooted phylogenetic analyses of genomes. Unrooted analyses have not fully supported this division, drawing attention to the importance of rooted trees of life.