Proso millet is an important short-duration crop that adapts well to varied climatic conditions and is grown worldwide for food, feed and fodder purposes. Owing to a lack of genetic improvement, the crop has experienced no yield improvement and provides low income to farmers. In this study, 200 accessions of proso millet originating in 30 countries were evaluated in two rainy seasons to assess phenotypic diversity for morpho-agronomic and grain nutritional traits and to identify high grain-yielding and grain nutrient-rich accessions. Proso millet diversity was structured by geographical region, by country within region, and by racial group. Race patentissimum showed high diversity and ovatum low diversity, and diverged widely from each other. The lowest divergence was observed between races compactum and ovatum. Eighteen high grain-yielding, 10 large-seeded, and 26 two or more grain nutrients-rich accessions were identified, and highly diverse pairs of accessions within and between trait groups were identified. They included IPm 9 and IPm 2661 for high grain yield and large seed size; and IPm 2069, IPm 2076, and IPm 2537 for high Fe, Zn, Ca, and protein contents. IPm 2875 had a seed coat that is readily removed by threshing. This study provides valuable information to proso millet researchers about agronomic and nutritional traits in accessions that could be tested for regional adaption and yield for direct release as cultivars, and could be used in breeding for developing high grain-yielding and nutrient-rich cultivars.