Abstract: This study aims to determine the radionuclide levels in seafood harvested from Nantong coastal waters and to assess associated health risks from seafood consumption. A total of 26 samples encompassing 10 species were collected in February and March 2022, January and April 2023, January and April 2024, and February and April 2025, including Palaemon gravieri, Cynoglossus semilaevis, Portunus trituberculatus, Meretrix meretrix, Larimichthys polyactis, Neopyropia yezoensis, Lophius litulon, Pampus argenteus, Scomberomorus niphonius, and Oratosquilla oratoria. The activity levels of ⁴⁰K, ²²⁶Ra, ²³²Th, ²³⁵U, ²²⁸Ra, ⁵⁸Co, ⁶⁰Co, ^110mAg, ¹³⁴Cs and ¹³⁷Cs in the samples were analyzed. The changes in radionuclide activity levels across different years were compared, and the committed effective doses and lifetime cancer risks were estimated. The results showed that among the artificial radionuclides, only ¹³⁷Cs was detected, ranging from 0.017 to 0.132 Bq/kg. A slight increase in ¹³⁷Cs activity concentration was observed in 2024–2025 compared to 2022–2023. The activity concentration ranges for the natural radionuclides ⁴⁰K, ²²⁸Ra, ²³²Th, ²²⁶Ra, and ²³⁵U were 20.20–120.89, 0.06–1.20, 0.06–1.69, 0.03–0.63 and 0.025–0.053 Bq/kg, respectively. The total committed effective doses from ingestion of seafood ranged from 10.15 to 49.01 μSv, with ¹³⁷Cs contributing less than 0.10%. Lifetime cancer risk from radionuclides in seafood was estimated at 3.05×10⁻⁵ to 5.10×10⁻⁵, which falls within an acceptable range. The lifetime cancer risk from ¹³⁷Cs ranged from 2.78×10⁻⁸ to 4.46×10⁻⁸, which was considered negligible. From February 2022 to April 2025, the radionuclide activity levels in seafood from Nantong coastal waters remained at background levels. Although the activity of ¹³⁷Cs showed a slight increase during this period, it remained within the fluctuation range of the national radiation environmental quality monitoring results in 2022. This increase may be related to species-specific bioaccumulation factors for ¹³⁷Cs. Continuous monitoring, with careful control of species types and sizes, is recommended. Furthermore, ²²⁸Ra was identified as a significant contributor to the internal dose from ingestion of these radionuclides. Overall, as of April 2025, the health risk from consuming seafood harvested from Nantong’s coastal waters is low.