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As a citizen‐based research project in an elementary school, we examined the life‐history traits of the endangered mud snail Batillaria multiformis in Mutsu Bay, one of the species northernmost populations in Japan. The snails occurred widely in the intertidal zone in summer, but they were densely aggregated near the low‐tide line from autumn to winter. Shell lengths (SLs) ranged from 1.2 to 49.5 mm, and one to four or more size cohorts occurred in the population. Batillaria multiformis spawned from August to September, settled in autumn, and grew to 4–6 mm SL the following spring. The snails grew to 5–10, 10–15, 15–20, and 20–25 mm SL in the second to fifth autumns, respectively. The highest growth rates were from July to August, while shell growth stopped from September to April. The snails became sexually mature at least 4–10 years (24.1 to ca. 50 mm SL) after settlement, at which time shell growth stopped and a callus knob was formed. Batillaria multiformis in Mutsu Bay exhibited much larger shell sizes and delayed maturation compared to southern populations. Shell growth rates differed in the summers of 2014, 2015, and 2016 (+0.69, +1.11, and +4.59 mm SL 30‐d−1, respectively), corresponding to the mean sediment temperatures in August (22.8, 24.5, and 26.4°C, respectively). These results indicate that thermal conditions constitute an important determinant of annual shell growth in the northern limit population of B. multiformis. Ongoing climate change can lead to changes in life‐history traits of macroinvertebrates, including mud snails, particularly near their northern distribution limits.