The effect of irrigation regimes and N rates on corn (Zea mays L.) yield and redroot pigweed (Amaranthus retroflexus L.) growth in sandy soils is not completely understood. Therefore, field experiments were conducted in a semiarid region of Iran in 2010 and 2011 to determine the influence of N rates and irrigation regimes on corn and redroot pigweed leaf area index (LAI), dry matter and grain yield, N uptake, irrigation water productivity (IWP) and N leaching loss. The experiment was conducted using a randomized complete‐block design with a split‐factorial arrangement of treatments in four replicates. The main plots were subjected to either a low (L) or a full (F) irrigation regime. The subplots were arranged in a factorial scheme with four N rates (0, 150, 300, and 450 kg N ha−1) and two levels of redroot pigweed interference with corn (weed free, CW0; and weedy, CW1). The results indicated that the more N was applied, the higher were corn grain yield losses in both irrigation regimes due to weed interference. The difference in corn grain yield between CW0F and CW1F treatments was 400 and 1800 kg ha−1 at N0 and N450 levels, respectively. In contrast, the difference between CW0L and CW1L reached 300 and 800 kg grain ha−1 at N0 and N450 levels, respectively. Based on the information gained in this study, we do not recommend increasing inputs (water and N), especially in sandy soils, to diminish the redroot pigweed/corn competition.