Assessments of yield potential, yield gap, and water productivity are critical to crop management. We assessed corn yield potential, yield gaps, and boundary water productivity for western part of the US Corn Belt using the Hybrid-Maize simulation model (http://www.hybridmaize.unl.edu/). The assessments were based on recent years, factual cropping information (including hybrid maturity, planting date, plant population, irrigation amount), basic soil properties, and multiple-year daily weather data. In this region, corn yield potential under irrigation was simulated to be 15.4±0.3 Mg ha^-1 (at 15.5% moisture content), while actual yield in farmers’ fields fluctuated around 12 Mg ha^-1 in recent years. Current actual yield is about 78% of the yield potential with a yield gap about 3.4 Mg ha^-1 in irrigated systems. Using simulated corn yield in both irrigated and rainfed systems, and reported total water input (including growing rainfall and irrigation), crop boundary water productivity, defined as yield potential per unit of total water input, was estimated to be 27.7±1.8 kg ha^-1 mm^-1 with a yield function Y = (X-100)*27.7, where Y is yield in kg ha^-1, X water input in mm. Average water productivity,  a useful benchmark for crop management, was estimated to be 19.3 kg ha^-1 mm^-1. The implications on crop management, particular on irrigation, are discussed based on the assessment of yield potential, yield gap and water productivity.