Nitrate is a common contaminant of drinking water. Due to its adverse health effects, this study aimed to determine nitrate levels in six southern districts of Tehran. A total of 148 samples were taken from tap waters. In 84.46% (n = 125) of the samples, the nitrate concentration was below national and WHO limits (50 mg/L); however, 15.54% (n = 23) were in violation of the criteria. The total mean concentration of nitrate was 36.15 mg/L (±14.74) ranging from 4.52 to 80.83 mg/L. The overall hazard quotient (HQ) for age groups were ordered as Children (1.71) > Infants (1.24) > Teenagers (1.2) > Adults (0.96). In all districts, the HQ values for infants and children groups were greater than 1, indicating potential adverse health risks. In teenagers age group, only the HQ estimations of districts 10 (HQ = 0.93) and 11 (HQ = 0.74) were lower than 1 and in adults age group, the estimated HQ values for districts were lower than 1 with the exception for district 19 (HQ = 1.19). The sensitivity analysis (SA) showed that nitrate content plays a major role in the value of the assessed risk.