A novel purple sulfur bacterium, strain AX1YPET, was isolated from marine sediments sampled at 47m depth in Callao Bay, Perú. Strain AX1YPE grew anaerobically, synthesizing bacteriochlorophyll a and carotenoid pigments of the spirilloxanthin series. Cells were Gram-stain-negative rods and actively motile by a polar flagellum. Strain AX1YPE was able to grow photolithoautotrophically with sulfide and thiosulfate as electron donors. This new phototrophic organism utilized ammonium salt, N2, urea and glutamate as nitrogen sources. Strain AX1YPE had a DNA base composition of 63.9mol% G+C. Analysis of the 16S rRNA gene sequence indicated that strain AX1YPE clusters in a separate branch within the genusAllochromatium of the family Chromatiaceae. Strain AX1YPE showed 16S rRNA gene sequence similarities of 98.2% with Allochromatium vinosum DSM 180T and Allochromatium minutissimum DSM 1376T, 98.1% with Allochromatium phaeobacterium JA144T, 97.3% withAllochromatium renukae DSM 18713T and 96.8% with Allochromatium warmingiiDSM 173T. DNA–DNA hybridization values to the type strains of its closest relatives, A. vinosum and A. minutissimum, were 59 and 64%, respectively. The predominant fatty acid of strain AX1YPETwas C18:1ω;7c and it notably possessed C20:1 as a minor component. PCR-based molecular typing (Box A1R and randomly amplified polymorphic DNA) produced a unique banding pattern for strain AX1YPET in comparison with the type strains of A. vinosum and A. minutissimum. Based on data from this polyphasic taxonomic study, which also includes average nucleotide identity comparison of five concatenated housekeeping genes, strain AX1YPET is considered to represent a novel species of the genus Allochromatium for which the name Allochromatium humboldtianum sp. nov. is proposed. The type strain is AX1YPET (=DSM 21881T=KCTC 15448T).