Eighty-three specimens of the bythograeid crab Austinograea williamsi Hessler and Martin, 1989, were collected from hydrothermal vent sites of the Mariana Trough. Of these, seventy-two specimens were examined for this study, including thirty-five males, thirty-six females, and one of unknown sex. Heterochely was observed, with one dactylus blunter and larger (the crusher type) than the other and one dactylus sharper and smaller (the cutter type) than the other. Females had cutters for both chelipeds, except one large female. Sixty percent of males (fifteen specimens) had a right crusher and a left cutter, and the others had two cutters. Morphometric analyses of the chela, abdomen, and carapace indicated male chelipeds were proportionally larger than female chelipeds, while female abdomens were proportionally larger than male abdomens. This sexual dimorphism was considered to be a secondary sexual characteristic related to different reproductive patterns between sexes. Additionally, using SEM, we studied the morphology of gonopods (first and second pleopods in males), which are systematically diagnostic for crabs.