Adaptogens are bioactive compounds that enhance the body's resilience to physical, chemical, and biological stressors, supporting homeostasis and overall well-being. Their beneficial effects are mediated through modulation of key stress-response pathways, including regulation of the hypothalamic–pituitary–adrenal (HPA) axis, attenuation of pro-inflammatory cytokines (e.g., TNF-α, IL-6), activation of antioxidant defense systems (e.g., Nrf2 pathway), and preservation of mitochondrial function. While traditionally associated with plants such as Panax ginseng, Eleutherococcus senticosus, Rhaponticum carthamoides, Rhodiola rosea or Schisandra chinensis, compounds with adaptogenic properties are increasingly recognized in a broader range of plant sources, being byproducts of the agri-food industry. This systematic review examines Spanish agri-food byproducts, including grape peel, olive tree leaves, citrus peels, apple peel, cacao residues, and tomato and beetroot pulp, as promising sources of adaptogenic compounds for functional food development. The review focuses on key bioactives such as resveratrol, hesperidin, theobromine, quercetin, oleuropein, betalains, and lycopene, summarizing their occurrence, bioactivity profiles, and potential roles in modulating neuroinflammation, metabolic disorders, arthritis, and stress-related conditions such as adrenal fatigue. The ;iterature was critically evaluated to identify both compositional data and evidence of adaptogenic effects or bioactivity-related adaptogenic effects in vitro and in vivo. The findings highlight the potential of these byproducts to contribute to the formulation of functional foods and beverages for the support of cognitive health, metabolic balance, and inflammatory control, while advancing circular economy practices within the Spanish agri-food sector.