The cosmopolitan oligochaete worm, Nais elinguis, is common to fresh and brackish water habitats. This species was found while investigating the limnology of two rivers alongside the Iranian coasts and has not been reported in the Iranian freshwater fauna and Caspian Sea before. N. elinguis was collected bimonthly from Cheshmehkileh and Sardabrood estuaries using a Van Veen grab (0.03 m²) and Surber (0.1 m², 0.2 mm-mesh size) with three replicates from 6 stations from November 2014 through September 2015. The results of temporal distribution showed that the highest and lowest density and biomass of this species were in January (242.2±84.45 ind m^-2 and 0.105±0.035 g m^-2) and in September (33.35±23.5 ind m^-2 and 0.010±0.006 g m^-2) respectively which were significantly different (p<0.05). Among the sampling stations,  station (S1) within the river with freshwater showed higher density and biomass (273.5±87.35 ind m^-2 and 0.116±0.036 g m^-2) than riverine brackish station (S2) within the mouth (39.8±22.6 ind m^-2 and 0.013±0.006 g m^-2) and marine brackish station (S3) (0±0 ind m^-2 and 0±0 g m^-2) which were significantly different (p<0.05). Density and biomass of this species in Cheshmehkileh River and estuary were more than that in Sardabrood. A significant correlation (p<0.05) between the density and biomass of N. elinguis with environmental variables was found.