At harvest, leafy vegetables undergo profound metabolic changes, causing oxidative stress and accelerating senescence, compromising their nutritional, nutraceutical, and organoleptic quality. These physiological modifications involve the degradation of key metabolites, affecting both primary and secondary metabolic pathways. Storage conditions play a crucial role in modulating these postharvest alterations, influencing metabolic responses related to oxidative stress, nutrient storage, and overall tissue integrity. Understanding these changes is essential for optimizing postharvest processing strategies and extending shelf life while maintaining the product quality. This study investigates the postharvest metabolic responses of Brassica rapa L. subsp. sylvestris (Friariello Napoletano), ecotype Sessantino, stored at 4 °C and 10 °C for 2 or 20 days. The results show that fresh samples (control) had higher levels (by over 50%) of sugars, proteins, chlorophylls, reduced ascorbate, and relative water content (RWC) compared to stored samples. Storage caused a decrease in sucrose and starch (by 75%), while the presence of several amino acids, including asparagine and glutamine, increased by more than 100% in stored samples. The increase in amides is due to amino acid catabolism, which releases carbon skeletons (acetate and oxalacetate), but also ammonia, which must be re-organicated to avoid further metabolic disturbances. Postharvest senescence was also evident after 20 days at 10 °C, showing a reduction in chlorophylls (by 80%) and ascorbate (by 85%), both of which are essential for preserving photosynthetic efficiency and antioxidant capacity. Interestingly, the tocopherol content increased by over 200%, probably due to the phytol released during chlorophyll degradation. This suggests that tocopherols act as protective molecules against oxidative stress, mitigating cell damage induced by storage conditions. Overall, prolonged cold storage, particularly at 10 °C, induces substantial metabolic changes in Brassica rapa L., reflecting stress adaptation mechanisms and marked changes in primary and secondary metabolism. Understanding these biochemical changes provides valuable indications for improving postharvest handling strategies of leafy vegetables, with the aim of prolonging shelf life while preserving nutritional and functional quality.