Activity concentrations of natural radionuclides, artificial radiocaesium, and soil-to-plant transfer factor have been evaluated in soils and plant (Amaranthus hybridus) samples from eighteen locations in south-western Nigeria with the aid of a co-axial type, High Purity Germanium (HPGe) detector. The mean specific activities of 40K, 226Ra, 232Th and 137Cs in the soil samples were 393.73 Bq kg−1, 52.91 Bq kg−1, 76.79 Bq kg−1 and 1.44 Bq kg−1, respectively, while the mean specific activities of 40K, 226Ra, and 232Th in plant samples were 3,271.66 Bq kg−1, 25.88 Bq kg−1, and 19.90 Bq kg−1, respectively. 137Cs was found to be below detection limit in the plant samples from all locations of the study area. The radium equivalent concentration, the external and internal hazard indices were estimated, and ranged from 28.47 to 701.53 Bq kg−1, 0.08 to 1.89, and 0.10 to 2.33, with mean values of 191.34 Bq kg−1, 0.52, and 0.66, respectively. The mean soil-to-plant transfer factors for 40K, 226Ra and 232Th were 26.58, 0.62, and 0.39, respectively. The mean absorbed dose and the mean annual outdoor effective dose equivalent in soil samples were 86.44 nGy h−1 and 0.1060 mSv y−1, respectively. The mean annual effective dose equivalent for the study area is higher than the world average (0.07 mSv y−1) and international recommended standards of 0.1 mSv y−1 recommended by World Health Organization (WHO). The excess lifetime cancer risk (ECLR) ranged from 7.35 × 10−5 to 1.36 × 10−3, with a mean of 3.71 × 10−4. This value is higher than the world average of 2.9 × 10−4 reported by United Nations Scientific Committee on the Effects of Atomic Radiation UNSCEAR. The ELCR is a function of environmental geology and 40K has very high soil-to-plant transfer factor compared to other radionuclides in the samples. Regression analysis showed that there was no linearity in the relationship between activity concentration of radionuclides in soil and in plant. This shows that soil-to-plant transfer factor is not a function of soil radioactivity.