Corynebacterium (/kɔːˈraɪnəbækˌtɪəriəm, -ˈrɪn-/) is a genus of Gram-positive bacteria and most are aerobic. They are bacilli (rod-shaped), and in some phases of life they are, more specifically, club-shaped, which inspired the genus name (coryneform means "club-shaped").

They are widely distributed in nature in the microbiota of animals (including the human microbiota) and are mostly innocuous, most commonly existing in commensal relationships with their hosts. Some, such as C. glutamicum, are commercially and industrially useful. Others can cause human disease, including, most notably, diphtheria, which is caused by C. diphtheriae. Like various species of microbiota (including their relatives in the genera Arcanobacterium and Trueperella), they are usually not pathogenic, but can occasionally capitalize opportunistically on atypical access to tissues (via wounds) or weakened host defenses.

The genus Corynebacterium was created by Lehmann and Neumann in 1896 as a taxonomic group to contain the bacterial rods responsible for causing diphtheria. The genus was defined according to morphological characteristics. Based on studies of 16S rRNA, they have been grouped into the subdivision of Gram-positive Eubacteria with high G:C content, with close phylogenetic relationships to Arthrobacter, Mycobacterium, Nocardia, and Streptomyces.

The term comes from Greek κορύνη, korýnē 'club, mace, staff, knobby plant bud or shoot' and βακτήριον, baktḗrion 'little rod'. The term "diphtheroids" is used to represent corynebacteria that are nonpathogenic; for example, C. diphtheriae would be excluded.^{[citation needed]} The term diphtheroid comes from Greek διφθέρα, diphthérā 'prepared hide, leather'.

Comparative analysis of corynebacterial genomes has led to the identification of several conserved signature indels (CSIs) that are unique to the genus. Two examples of CSIs are a two-amino-acid insertion in a conserved region of the enzyme phosphoribose diphosphate:decaprenyl-phosphate phosphoribosyltransferase and a three-amino-acid insertion in acetate kinase, both of which are found only in Corynebacterium species. Both of these indels serve as molecular markers for species of the genus Corynebacterium. Additionally, 16 conserved signature proteins, which are uniquely found in Corynebacterium species, have been identified. Three of these have homologs found in the genus Dietzia, which is believed to be the closest related genus to Corynebacterium. In phylogenetic trees based on concatenated protein sequences or 16S rRNA, the genus Corynebacterium forms a distinct clade, within which is a distinct subclade, cluster I. The cluster is made up of the species C. diphtheriae, C. pseudotuberculosis, C. ulcerans, C. aurimucosum, C. glutamicum, and C. efficiens. This cluster is distinguished by several conserved signature indels, such as a two-amino-acid insertion in LepA and a seven- or eight-amino-acid insertions in RpoC. Also, 21 conserved signature proteins are found only in members of cluster I. Another cluster has been proposed, consisting of C. jeikeium and C. urealyticum, which is supported by the presence of 19 distinct conserved signature proteins which are unique to these two species. Corynebacteria have a high G+C content ranging from 46-74 mol%.

The principal features of the genus Corynebacterium were described by Collins and Cummins, for Coryn Taylor in 1986. They are gram-positive, catalase-positive, non-spore-forming, non-motile, rod-shaped bacteria that are straight or slightly curved. Metachromatic granules are usually present representing stored phosphate regions. Their size falls between 2 and 6 μm in length and 0.5 μm in diameter. The bacteria group together in a characteristic way, which has been described as the form of a "V", "palisades", or "Chinese characters". They may also appear elliptical. They are aerobic or facultatively anaerobic, chemoorganotrophs. They are pleomorphic through their lifecycles, they occur in various lengths, and they frequently have thickenings at either end, depending on the surrounding conditions.

Some corynebacteria are lipophilic (such as CDC coryneform groups F-1 and G, C. accolens, C. afermentans subsp. lipophilum, C. bovis, C. jeikeium, C. macginleyi, C. uropygiale, and C. urealyticum), but medically relevant corynebacteria are typically not. The nonlipophilic bacteria may be classified as fermentative (such as C. amycolatum; C. argentoratense, members of the C. diphtheriae group, C. glucuronolyticum, C. glutamicum, C. matruchotii, C. minutissimum, C. striatum, and C. xerosis) or nonfermentative (such as C. afermentans subsp. afermentans, C. auris, C. pseudodiphtheriticum, and C. propinquum).

While some Corynebacterium species possess teichoic acid in their cell walls, the plant pathogenic species do not.The cellular envelope of Corynebacterium has esterified short-chain alpha-branched and beta-hydroxy fatty acids that are covalently bound to the peptidoglycan wall, forming a mycolyl-peptidoglycan complex. This contributes to the stress resistance and pathogenicity of Corynebacterium. Most phytopathogenic Corynebacterium cause infections upon injury or wounding of the plant and can cause systemic infections as well. C. fascians is found in perennials and can cause leaf and bulb distortion, bud deformity, and proliferation wilt. C. flaccumfaciens ssp. Flaccumfaciens in field beans can cause wilting, C. flaccumfaciens ssp.  Betae in beets can cause silvering of the leaves and wilting, C. ilicis in American holly can cause branch blight, and C. iranicum in wheat can cause yellow slime of the leaves, inflorescences, and leaf spots. C. michiganense ssp. Michiganense in tomato peppers causes wilting and fruit spots, C. michiganense ssp. Insidiosum in alfalfa causes wilting and stunting, and C. tritici in wheat causes yellow slime of leaves and inflorescences. Some Corynebacterium species are considered zoonotic agents which can be transmitted to humans by contact with diseased animals.