Faxonius neglectus neglectus (Ringed Crayfish) have been characterized as an “extraregional” invader whose range expansion has threatened native crayfish. We determined length–weight and body part–length relationships for Ringed Crayfish using reference specimens collected from their native range in the Ozark Highlands. Analysis of covariance and estimated marginal means demonstrated males are heavier and possess larger chelae than females of equal size. In addition, males' maximum values of total length, dry weight, and chela length/width were all larger than those of females. Sexual dimorphism in this species necessitated sex-specific length–weight regressions in addition to a combined model. Length–weight relationships were all significant (P < 0.001) with coefficients of determination (R²) ≥ 0.971. Both male and female Ringed Crayfish exhibited positive allometric growth (b > 3). Regression equations predicting Ringed Crayfish total length based on carapace length, abdomen length, chela length, and palm width were significant (P < 0.001) with R² ≥ 0.925. Length–weight and body part–length equations will facilitate ecological studies of Ringed Crayfish and provide comparative data for studies investigating mechanisms of competitive interactions among sympatric crayfish species. These results contribute to the growing number of species-specific studies evaluating patterns of growth and sexual dimorphism in crayfish.