Abstract <p>—This study describes the specific features of the molt cycle, growth, sex structure, and size structure of the helmet crab <i>Telmessus cheiragonus</i> for the first time. This species inhabits the waters off the Shantar Islands and western Kamchatka in the northern Sea of Okhotsk. On the western Kamchatka shelf, the densest male aggregations were found at the depths of 13 to 55 m (21.3 ± 3.2 m) at temperatures ranging from 2.8 to 6.5°C (4.8 ± 0.3°C), while females aggregated at the depths of 16 to 19 m (17.3 ± 0.9 m) at temperatures ranging from 4.3 to 5.2°C (4.7 ± 0.3°C) in June. Near the Shantar Islands, the densest aggregations were observed at depths ranging from 13.2 to 22.8 m (18.0 ± 4.8 m) and temperatures ranging from 8.3 to 15.5°C (11.9 ± 3.6°C) in September. The asymptotic carapace widths (CW_inf) of the female and male groups on the western Kamchatka shelf were 66.3 and 82.6 mm, respectively. This was lower than the 86.73 mm observed in males on the shelf north and northwest of the Shantar Islands. Helmet crabs off western Kamchatka molt in May–June, whereas a longer molting period is assumed for the crab aggregation near the Shantar Islands, from May to August. The estimated average size at maturity (CW_mat) was 40.6 mm for females and 40.3 mm for males. Slower development and a later onset of physiological maturity (CW_mat of 42.1 mm) were observed in male crabs from the Shantar aggregation.</p>