Valorizing neglected crop byproducts through eco-friendly techniques has gained attention as underutilized crops offer new sources of bioactive components. This study examined the chemical composition, phytochemicals, and antioxidant activities in six fractions of the underutilized chayote (Sechium edule). Each fraction was microwave-dried and extracted with ultrasound assistance. Shoots were high in ash and fiber, while pulp had high moisture and carbohydrates, and seeds were rich in fat and protein. Shoots, leaves, roots, and peels had high mineral levels, particularly Ca, K, Mg, P, Fe, and Zn. Shoots were also high in phenolics, flavonoids, and ascorbic acid, while leaves had a high carotenoid content. Polyphenol compounds such as myricetin and catechin were abundant in shoots, which exhibited the highest antioxidant activity. Chayote shoots, roots, and leaves are abundant in bioactive compounds that can be utilized in food and, therefore, offers an opportunity for utilization of a currently neglected plant byproduct.