Shortage of irrigation water, being a crucial problem in arid and semiarid regions, makes it necessary to use treated wastewater in agriculture. A field experiment was conducted to investigate the effect of treated municipal wastewater (TMW) on the yield and fiber quality of cotton (Gossypium hirsutum L.) (Mehr variety). Treated municipal wastewater was supplied from Shahrake Ghods sewage treatment plant in Tehran. The experimental treatments were: irrigation of the crop through surface irrigation by different mixtures vs. intervals of freshwater and TMW. Two additional treatments, namely: the cotton crop irrigated with freshwater vs. TMW which were considered as control. The experimental design was a randomized complete block one, consisting of eight treatments and three replications. The results showed that cotton yield, number of bolls per m2, leaf area index (LAI) and plant height were significantly greater when the crop irrigated with TMW rather than with freshwater. The yields in TMW vs. freshwater treatments were about 2200 and 780 kg of lint ha-1, respectively. Also, there was no significant detrimental effect observed on the characteristics of cotton fiber quality when the crop irrigated with TMW.