Corn (Zea mays L.) planted in narrow row spacing (<30-inch) with high plant population has become a recent interest in the US Corn Belt to increase grain yield. We evaluated the impact of row spacing and plant population on corn grain yield across 22 site-years in Iowa from 2009 to 2018. Experiments were designed as a split-plot with two row spacings, 20- and 30-inch, as the main-plot and three to four plant populations, ranging from 30,000 to 42,000 plants acre^–1, as subplot. Grain yield was affected in 73% of the site-years: 13 site-years by row spacing, six site-years by plant population, and 2 site-years by the interaction of both. Corn in 20-inch rows yielded 5 to 19 bu acre^–1 more in 11 site-years and 10 to 14 bu acre^–1 less in two site-years compared to 30-inch rows. In both 20- and 30-inch row spacings, corn yield decreased linearly at 0.4 to 1.7 bu acre^–1 per thousand increase in plant population in four site-years and responded quadratically with peak yield at around 36,000 plants acre^–1 in two site-years. However, corn yield increased linearly at 2.0 to 3.1 bu acre^–1 per thousand increase in plant population only when planted in 20-inch row in two site-years. When all 22 site-years were combined, yield was only affected by row spacing. Corn in 20-inch rows produced similar yield under low yielding environments and 8 to 10 bu acre^–1 more yield under high yielding environments compared to 30-inch row spacing. Results suggest that farmers should move to 20-inch row spacing at yield levels greater than 235 bu acre^–1.