The by-product of walnut oil (Juglans regia L.) extraction is a press cake rich in polyunsaturated fatty acids and other bioactive compounds. From this cake, walnut flour is obtained by a milling process. The composition and a physicochemical characterization of walnut flour was performed: proximal composition, mineral content, and fatty acid and amino acid profiles were measured. Besides, antioxidant capacity and water and oil holding capacities were determined. Walnut flour has 55% of lipids with an optimum w6/w3 ratio, a good lysine/arginine ratio, and high levels of antioxidants that contribute to its oxidative stability, the estimated shelf life being 16 months. In regards to interaction with other ingredients, walnut flour retained 258 and 70% (w/w) of water and oil, respectively. Therefore, these results show that walnut flour is a good source of micro- and macronutrients, compared to flours commonly used in breadmaking. Also, walnut flour has good technofunctional properties and thus its incorporation could improve the nutritional and technological characteristics of new bakery products.