Abstract Objective Suckermouth armored catfishes (SACs; Loricariidae) have invaded water bodies outside their native range globally. Population control methods, including removals and genetic biocontrols, may be enhanced through greater knowledge of sex discrimination, sex ratios, and potential sex bias among control methods. Methods We investigated sexual dimorphism and population sex ratio for an invasive SAC population of Hypostomus sp. in the upper San Marcos River, Texas, USA, as a means of advancing ongoing management of the population. Result Among 385 specimens obtained at random during a planned dewatering of a 100-m reach of the river, the sex ratio was biased toward females (proportion of females = 0.60). Morphometric analyses of 105 females and 108 males based on a generalized boosted classification model revealed elongated dorsal and anal fins in males correctly classified sex for 83% of individuals (95% CI = 77–88%). Seven independent observers used results from the generalized boosted classification model to assign sex to 30 fish, with average accuracy of 72% (range = 43–83%). The accuracy of the model and validation exercises were highest for fish >240 mm total length, the estimated length of reproductive maturity. Conclusion Morphometrics based on dorsal- and anal-fin sizes can be used to infer sex externally from hundreds of SACs removed per year to evaluate long-term population management success.