Increased corn (Zea mays L.) seed costs and hybrids with greater stress tolerance than in the past make it important to know if the optimum plant density for corn grain yield differs with hybrid relative maturity (RM) or row width. In 2009 and 2010 at two locations in southern Minnesota, agronomic responses of corn to plant densities ranging from 40,700 to 108,700 plants ha-1 were evaluated for 95-, 101-, and 105-d RM hybrids in 51- and 76-cm row widths. Stalk diameter, intercepted photosynthetically active radiation (IPAR) and leaf area index (LAI) at silking, root and stalk lodging, grain yield, and yield components did not differ with row width, and the response of these variables to plant density was not affected by hybrid or row width. Yield of the 105-d RM hybrid was 13% higher than that of the 95-d RM hybrid due to greater kernel number despite lower kernel weight, but was similar to the 101-d RM hybrid. There was a quadratic-plateau response of grain yield to plant density, with the plateau occurring at 84,500 plants ha-1. Net return was not affected by row width, hybrid, or 23 of 25 scenarios for seed cost and grain price. The economically optimum plant density was 62,200 plants ha-1 for US$350 per 80,000 seeds and US$120 Mg-1, and 87,000 plants ha-1 for US$150 per 80,000 seeds and US$280 Mg-1. These results demonstrate that grain yield can be maximized with mid- and late-RM hybrids and plant densities ≥84,500 plants ha-1 in either 51- or 76-cm rows.