Systematic studies were carried out to understand the distribution of natural radionuclides in sediments and radon in water in the riverine environs of Cauvery, one of the major rivers of South India. The activity of radionuclides in the sediment was measured by gamma ray spectrometry. The radon emanation from the sediment was measured by the sealed ‘can technique’ and the radon in the water was measured using the RAD-7 instrument. The mean values of 40 K, 226Ra, and 232Th in the sediment samples were found to be 297.3 ± 4.16 Bq kg−1, 75.1 ± 2.64 Bq kg−1, and 85.5 ± 2.62 Bq kg−1, respectively. The mean activity of radon, radon exhalation rate, and radium content were found to be 135.68 Bq m−3, 327.1 mBq m−1 h−1, and 133.03 mBq kg−1, respectively. The radon in the water ranged from 0.19 kBq m−3 to 1.40 kBq m−3. The hyper pure germanium gamma spectroscopy measured via 226Ra activity and the radon activity measured by the passive can technique showed good correlation. The mean value of radon in the water was within the internationally recommended level. The sediment was considered safe for the purpose of construction, except for some extreme values, and the water was deemed safe for drinking.