The effect of supplementation of grazing cattle with unconventional agro-industrial by-products on milk production and economic performance was evaluated in the Amazon region of Peru. Ten lactating cows were used in a simple crossover design with two periods of 21 days (11 days of adaptation and 10 days of measurements), and two treatments: conventional supplementation (rice polishing) and a mixture of unconventional agro-industrial by-products—MUABP (rice polishing, rice middling, cocoa hull, and coconut meal). Cattle supplemented with MUABP produced more milk than those fed the conventional supplement (10.2 vs. 8.8 kg/cow/day, p <0.001). No differences were found between the two treatments in protein, fat, or lactose content of milk (3.9%, 3.17%, 4.54% on average respectively; p ≥0.05). Daily weight gain with the MUABP treatment was 0.09 kg/day, while with conventional supplementation cow lost −0.04 kg/day (p =0.01). Body condition did not differ between treatments (p ≥0.05). Income due to supplementation with unconventional agro-industrial by-product was US $0.2 in comparison with only rice polishing. Cattle supplemented with MUABP improved milk production and their economic profitability.