The major histocompatibility complex is a diverse gene family that plays a crucial role in the adaptive immune system. In humans, the MHC class I genes consist of the classical loci of HLA‐A, ‐B, and ‐C, and the nonclassical loci HLA‐E, ‐F, and ‐G. In Platyrrhini species, few MHC class I genes have been described so far and were classified as MHC‐E, MHC‐F, and MHC‐G, with MHC‐G possibly representing a classical MHC class I locus while there were arguments about the existence of the MHC‐B locus in Platyrrhini. In this study, MHC class I genes were identified in eight common marmosets (Callithrix jacchus) and two brown‐headed spider monkeys (Ateles fusciceps). For common marmosets, 401 cDNA sequences were sequenced and 18 alleles were detected, including 14 Caja‐G alleles and 4 Caja‐B alleles. Five to eleven Caja‐G alleles and one to three Caja‐B alleles were detected in each animal. For brown‐headed spider monkeys, 102 cDNA sequences were analyzed, and 9 new alleles were identified, including 5 Atfu‐G and 4 Atfu‐B alleles. Two or three Atfu‐G and two Atfu‐B alleles were obtained for each of animal. In phylogenetic analyses, the MHC‐G and ‐B alleles from the two species and other Platyrrhini species show locus‐specific clusters with bootstrap values of 86% and 50%. The results of pairwise sequence comparisons and an excess of non‐synonymous nucleotide substitutions in the PBR region are consistent with the suggestion that Caja‐G and Atfu‐G may be classical MHC class I loci in the Platyrrhini species… But it appears that MHC‐B locus of the two Platyrrhini species shares features with both classical and nonclasical MHC class I loci. Our results are an important addition to the limited MHC immunogenetic information available for the Platyrrhini species. Am. J. Primatol. 77:527–534, 2015. © 2015 Wiley Periodicals, Inc.