Nitrogen (N) fertilizer application can affect competition with weeds that are common in many dry bean (Phaseolus vulgaris L.) production systems in developing countries. Field research was conducted to investigate the interaction between N fertilizer rate and redroot pigweed (Amaranthus retroflexus) density on bean seed yield. Experiments were conducted in 2009 and 2010 to determine the responses of two bean genotypes with different growth habit, semi-erect and erect, to different N fertilizer rates under no, low and high redroot pigweed pressure. Nitrogen was applied at rates of 0, 50, 100 and 200 kg N ha−1 and 0, 35, 70 and 140 kg ha−1 in semi-erect and erect bean growth habits, respectively. An increase in N fertilizer increased biomass, seeds m−2 and seed yield for both bean genotypes when grown under no and low weed pressure. Bean yield loss for the semi-erect growth habit ranged from 8 to 9% at 0 kg N ha−1 and 30 to 33% at highest N rate. Yield loss for the erect growth habit was 13–17% at 0 kg N ha−1 and 33–40% at the highest N rate. An increase in redroot pigweed density reduced bean yield especially at the highest N rate. The response surface model that was developed using the experimental observations indicated that optimal bean seed yield could be achieved at derived values of 120 and 84 kg N ha−1, respectively, in semierect and erect growth habits, while maintaining a redroot pigweed density of 2600–2800 plants h−1. Semi-erect bean growth habit could tolerate higher redroot pigweed densities compared to erect bean growth habit. We conclude that redroot pigweed should not exceed 2600–2800 plants ha−1 to optimize yield, and N rates of 120 and 84 kg ha−1 will maximize dry bean yield under moderately weedy conditions for the erect and semi-erect growth habits, respectively.

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