Agri-food waste (AFW) and by-products represent sources of phytochemicals, such as phenols and antioxidant compounds that can be used as functional ingredients in animal feed. In this study, a selection of AFW and by-products were collected and analysed for their nutrient composition. Fruit and vegetable waste (FVW), orange peel, strawberry, citrus pulp and Camelina sativa cake showed a total phenolic content ranging from 238.0 ± 4.24 to 1583.0 ± 154.35 mg TAE/100g. Grape marc also showed the highest AOC (15440.7 ± 2671.85 mg TE/100g). In all other samples, AOC ranged from 43.3 ± 3.17 to 1703.9 ± 391.07 mg TE/100g. After physiological extraction, total phenolic content values higher than 3000 mg TAE/100g were observed in FVW, grape marc and orange peel. Grape marc, C. sativa cake and orange peel had AOC values of over 5000 mg TE/100g. The digestibility of AFW and by-products ranged from 44.20 to 97.16%. The lowest digestibility value was observed in grape marc (44.2 ± 2.31%). In conclusion, the results obtained in this study indicate that AFW and by-products could be a source of bioaccessible phenols and antioxidant molecules as ingredients for monogastric compound feeds.