In the present study, radon concentrations were measured in 70 groundwater samples collected from Tuticorin district of Tamil Nadu, India. Radon activity concentration was found in the range of 0.07–40.70 Bq L^-1 with an average value of 5.13 Bq L^-1. Radon values were compared with USEPA and WHO. In this study radon activity trend is above the permissible limit may be due to influence of heavy rainfall. A good correlation is found with deeper borehole depth and lithology. Granitic terrain is found to have higher radon concentration.