<p>Allometric growth and secondary sexual characteristics are two important indicators of the morphological maturity and reproduction of crustaceans, but very limited information could be found on the male swimming crab <i>Portunus trituberculatus</i>. Screening based on secondary sexual characteristics thereby provides a scientific basis for selecting sexually mature male crabs in both pond farming and artificial reproduction processes. This study was conducted to investigate the allometric growth of morphological characteristics as well as the changes in secondary sexual characteristics during the period from juveniles to adults of pond-reared male <i>P. trituberculatus</i>. The results showed the following: (1) The univariate variance analysis showed that FFLC, MLC, FPL, and SPL increased significantly between August and September. The principal component analysis showed that FFLC, MLC, FPL, and SPL had higher loading values in the first principal component. (2) According to the allometric growth of ontogeny, five secondary sexual characteristics of male crabs were identified as FFLC, MLC, FPL, SPL, and TPL. The estimates of carapace width for the morphological maturity of males were equal to 91.03&#xa0;mm, and the maturity of male crabs was concentrated in August. (3) During growth and development, the proportion of blue markings on the cheliped gradually increased, and the <i>b</i> value (yellow–blue value) showed a significant decline. The proportion of blue markings of mature morphology was more than 8.91%, and the yellow–blue value was lower than −5.00. In conclusion, these results showed that FFLC, MLC, FPL, SPL, TPL, and cheliped color were considered six secondary sexual characteristics for the male <i>P. trituberculatus</i> between pre- and post-pubertal molting during the period from August to September, and cheliped color provides a simple, fast, and convenient biomarker to identify morphological maturity.</p>