The present work deals with assessment of baseline radionuclide concentration in marine organisms around selected coasts of Maharashtra, India. This baseline study highlights concentrations of natural and fallout radionuclides in finfish and shellfish species found in the surrounding seawater. Water and fish samples were collected, processed, and analyzed for 238U, 226Ra, 228Ra, 232Th, 40K, and 137Cs by high-resolution gamma spectrometry. 238U, 226Ra, 228Ra, 232Th, 40K, and 137Cs concentrations were found to be in the range of ≤1.5, ≤2.0, ≤0.4, ≤1.5, ≤8.0 to 258, and ≤0.2 Bq/kg, respectively. The seasonal study revealed the maximum concentration in premonsoon season. Activity concentrations were higher in pelagic fish than those in demersal fish. A higher concentration factor was reported in shellfish than in finfish. •Present work is first documentation on baseline radioactivity (natural and artificial) conducted around a proposed nuclear power plant along southwest konkan region for nuclear energy in future.•The results for all naturally occurring radionuclides were BDL except for K-40.•The concentration of 40K was found to be in the range of ≤8.0 to 258 Bq/kg.•Seasonal variation revealed maximum concentration in pre-monsoon compared to post-monsoon.•Maximum activity concentrations were found in pelagic fishes compared to demersal fishes.