The cotton boll weevil, Anthonomus grandis (Coleoptera: Curculionidae), is the major insect pest of cotton in the Americas. Although the demographic and dispersion patterns of the cotton boll weevil in the USA and Mexico were well characterized, in Brazil the information is scarce and does not include the main cotton‐producing regions of the country. Thus, our objective was to apply a phylogeographic approach sequencing a gene fragment of the cytochrome c oxidase subunit I (COI) to estimate the genetic diversity, identify the genetic strains, and describe the dispersion routes of the cotton boll weevil in the Brazilian territory. The COI marker revealed a low genetic diversity of the cotton boll weevil with only two haplotypes in Brazil. The haplotypes are genetically related to individuals from the eastern region of the USA and Mexico, which suggests a recent invasion event in Brazil. The genetic structure of COI haplotypes indicates a low dispersion of boll weevils between north/northeast and midwest/southeast regions of Brazil. The genetic drift and low gene flow during the insect expansion explain the COI haplotype distribution. The genetic structure of the cotton boll weevil in Brazil is novel information and provides valuable insight for the design of suitable pest management practices against this key pest species in the country.