Interest in the sustainable use of agri-food by-products has grown over the years. The goal of this work was to determine the content in fibers and proteins, as well as the amino acids' profile of residues obtained from the extraction process of bioactive molecules, which is normally discarded and considered as waste. For this, it was obtained the residues from the maceration extraction of: 1) kale (Brassica oleracea var. Acephala); 2) zucchini (Cucurbita pepo L.); and 3) onion peel (Allium cepa L.) by-products. The residues were separated, freeze-dried, and characterized (in terms of the content of total, soluble, and insoluble dietary fibers, as well as crude and total proteins), following standardized methods of the AOAC. Additionally, the amino acids’ profile was determined using chromatographic methods. The onion peel exhibited the highest content in fibers, i.e., 14.54 ± 0.37% of soluble dietary fibers and 54.61 ± 0.53% of insoluble dietary fibers. The crude protein and the total protein contents were significant in the kale (26.9 ± 0.2 and 23.34 ± 0.02 g/100g dw) and zucchini (30.67 ± 0.5 and 27.43 ± 0.03 g/100g dw) residues. These residues also showed the major total essential amino acids’ content with 82.21 ± 3.90 mg/100 g dw for kale residue and 98.55 ± 4.70 mg/100 g dw for zucchini residue, with phenylalanine and leucine as predominant amino acids. In terms of non-essential amino acids’ content, aspartic and glutamic acids were crucial in all the studied residues. The obtained results show the potential of the studied residues to be used as ingredients in the food industry, promoting the circular economy by transforming by-products into sources of fiber and amino acids.