Pressure on the food industry to reduce environmental impact has positioned the revalorization of by-products as a major strategy to address this effect. One strategy that could be useful in addressing this problem is solid-state fungal fermentation (SSSF). The aim of this study is to use SSSF on by-products (remaining cakes) from the plant-based beverage industry (tiger nut, carob, and rice) to increase protein content and antioxidant capacity. For this purpose, two strains of filamentous fungi (Aspergillus oryzae var. oryzae CBS 819.72 and Fusarium venenatum CBS 458.93) presenting Qualified Presumption of Safety status, were used as sources to produce mycoprotein. The protein content was determined by Kjeldahl method, Total Antioxidant Capacity (TAC), by ORAC and TEAC methods, and Total Polyphenols (TP) by the Folin-Ciocalteu method. The results showed a significant increase in proteic biomass in all fermented matrices, especially in the case of rice by-product with A. oryzae. Likewise, an increase in TAC was observed with notable improvements in all fermented samples, except for carob by-product, which reduced values in all the methods analyzed, except in ORAC. In conclusion, this process represents an effective strategy for the valorization of agri-food by-products, promoting the circular economy and sustainability.