Studies on the effect of nitrogen (N) application rates on lint yield and fiber quality in irrigated and rainfed cotton were conducted for two years. In 2013, cotton was planted in 48 plots. Twenty-four plots were irrigated and the other 24 pots were rainfed. Six N application rates (0, 39, 67, 101, 135, and 168 kg/ha) with four replicates were randomly assigned to the irrigated and rainfed plots. In 2014, five N treatments (0, 56, 112, 168, and 224 kg/ha) with four replicates were assigned to 20 irrigated plots. Effect of N application rates on cotton lint yield was significant in 2014 (p = 0.0196), but not in 2013. Yield showed a quadratic relationship with leaf N content in irrigated cotton in both 2013 (p = 0.0268) and 2014 (p = 0.0099). Correlation between leaf N and yield of rainfed cotton was not significant in 2013. Leaf N of irrigated cotton in 2014 had significant correlation with fiber length (p = 0.0037), UQL (p = 0.0001), and UHML (p < 0.0001). Yellowness was linearly related with leaf N content. Fiber strength showed a linear relationship with leaf N in 2013 rainfed cotton (p = 0.0495), a quadratic relationship with irrigated cotton in 2013 (p = 0.0231) and 2014 (p = 0.0365). Overuse of nitrogen fertilizer in cotton could result in loss of yield and fiber quality. When the fiber quality from irrigated cotton was compared with rainfed cotton, irrigation increased lint yield by 26% and fiber length by 2%.