To assess the environmental radioactivity levels and hence to determine the population exposure in Kutubdia Island, Bangladesh the sediment samples were analyzed by using gamma ray spectrometry. In those sediment samples the average activity concentration of natural radionuclides ²³⁸U, ²³²Th and ⁴⁰K were found 15.39±1.67, 38.35±0.76 and 475.51±21.15 Bq/kg respectively. The activity concentration of ²³⁸U and ²³²Th were less than the world average. But the activity concentration of ⁴⁰K was higher than the world average values. These results were used to calculate the radiological hazard parameters like radium equivalent activities (Ra_eq), representative level index (I_γr). Due to natural radionuclides in sediment, the effective annual outdoor dose in the study area was 0.07 mSv; which is within the accepted range of 0.07 mSv. The average value of radium equivalent activities (Ra_eq) and representative level index (Igr) were found to be 128.03±12.08 Bq/Kg and 0.96±0.09 Bq/Kg which were greater than the world average. The anthropogenic radionuclide ¹³⁷Cs was also found in few samples. The average activity concentration of ¹³⁷Cs was 1.64±0.20 Bq/kg which is lower than the world average. The obtained experimental data of this research work would be useful to assess the population exposure from radionuclides in sediment as well as base line data of natural radioactivity in this island.