The Proteobacteria are a major group of bacteria, including many important nitrogen fixing bacteria and pathogens. The name comes from the Greek god Proteus, who could change shape, and refers to the great diversity of forms found in this group, which is defined mainly in terms of RNA sequences. Proteobacteria have cell walls composed mainly of lipopolysaccharides, giving them gram-negative stains. Many have flagella, while others may move about through bacterial gliding. Most are anaerobic. Some, called purple bacteria, are capable of photosynthesis, which uses hydrogen sulfide, sulfur, or hydrogen as an electron donor, and so does not produce oxygen.

The proteobacteria are divided into five major groups, sometimes treated as classes, which are labelled with the Greek letters alpha through epsilon. Some of these may be paraphyletic. The mitochondria found in eukaryotic cells may represent reduced endosymbiotic proteobacteria as well.