Kadal flour is a fermented grain flour that is produced by naturally fermenting grains in water, washing, drying, milling, and sieving. This study evaluated the effect of natural fermentation and fermented Bambara groundnut supplementation on the nutritional quality of kadal flour produced from whole and decorticated millet grains. Millet and Bambara groundnut were fermented for 72 h, processed into flour, and blended at millet-to-Bambara groundnut ratios of 80:20, 70:30, and 60:40, for both whole millet and Bambara groundnut and decorticated millet and Bambara groundnut kadal flour blends, with control samples composed of 100% whole and 100% decorticated millet kadal flours, respectively. Proximate composition, energy value, amino acid profile, and vitamins B₁ and B₉ were determined using standard methods. Fermentation increased moisture and fibre contents but reduced protein, ash, carbohydrate, energy value, and as well as most of the essential and non-essential amino acids, with significant decrease in decorticated millet kadal flour. However, the fermentation process resulted to increase in phenylalanine, threonine, and tyrosine contents. Supplementation with fermented Bambara groundnut flour significantly improved protein, fat, ash, fibre, carbohydrate, energy values, amino acid contents, and vitamins B₁ and B₉, with higher inclusion levels producing greater increases. Whole millet–Bambara groundnut kadal flour blends had higher contents than their decorticated grain counterparts. The sample with 60% whole millet and 40% whole Bambara groundnut) recorded the highest nutrient contents and energy value of 380.11 kcal/100 g. It was concluded that production of kadal flour from blends of whole millet and Bambara groundnut could help in reducing the problem of malnutrition particularly in sub-Saharan Africa. Moreover, decortication of grains destined for kadal flour processing is not recommended as it leads to greater loss in nutrients.