Data on the extent to which the sexes may differ in their phenotypes are critical for a full understanding of the biology and management of any species. We previously quantified behavioral differences and vocal similarities between genetically-sexed Australasian Gannets (Morus serrator). Here, we quantify size monomorphism and plumage dichromatism in this socially monogamous, colonial seabird. In comparison with other sulids, the Australasian Gannet is characterized by low sexual dimorphism indices in various size metrics, and most physical dimensions are statistically similar between adult female and male gannets. In contrast, we found indications of sexually dichromatic plumage traits in the melanin-based, rusty head plumage and in the black-and-white tail feathers. To our knowledge, these findings constitute the first evidence of melanin-generated sexual plumage dichromatism in a size monomorphic seabird species. Using opsin-sequencing, we also confirm that the Australasian Gannet is a visually violet-sensitive species, for which the detection of both gross differences in feather reflectance, and long-wavelength based plumage dichromatism, should be perceptually feasible. However, because of the extensive overlap between females and males in the size and chromatic traits detected here, and in the behavioral and vocal displays reported in previous studies, we advocate for the use of genetic techniques for sex identification in this gannet species.