The grapefruit (Citrus x paradisi) is the sixth most consumed citrus fruit worldwide. It's known for its tangy flavor and reddish-pink pulp, primarily consumed as juice or pulp. The peel and mesocarp are discarded, but they contain valuable antioxidant compounds and biologically relevant substances. This study aimed to identify and quantify phenolic compounds in various grapefruit parts and evaluate their antioxidant capacity. Three methodologies (TBARS, DPPH, and OxHLIA assays) were used to assess antioxidant potential. The identification and quantification of the phenolic compounds were performed by HPLC-DAD/ESI-MS. The grapefruit was divided into three parts: peel, mesocarp, and pulp, each of which underwent a hydroethanolic extraction to obtain extracts for phenolic composition identification and antioxidant potential evaluation. The mesocarp was demonstrated to have the highest amount of phenolic compounds (48.4 mg/g of extract), followed by the peel (36.7 mg/g of extract). Moreover, the peel extract exhibited the best result in all three antioxidant activity methods performed. The higher antioxidant activity of the peel compared to the other extract could be attributed to its higher concentration in naringenin and its derivatives, particularly O-triglycosyl naringenin and acetyl naringenin, being these the phenolic compounds found in higher quantities in the peel. Based on the obtained results, both waste and pulp extracts of grapefruit showed to be promising natural ingredients that could be applied in different industrial sectors.