Citrus fruits are a rich source of bio-functional compounds with various and well-proven health properties. We focused our attention on an ancient Mediterranean species, Citrus lumia Risso. The aim of this work was to investigate the polyphenol content and biological activities of C. lumia albedo extract by cell-free and cell-based assays. Fifteen polyphenols were quantified by LC-DAD-FLD analysis (flavonoids 89.34% and phenolic acids 10.66%) with eriocitrin and hesperidin as major components (52.81% and 31.31%, respectively). These results were corroborated also by micromorphological observations, which showed clusters of needle-shaped crystals of hesperidin highlighted by TBO staining. C. lumia albedo extract revealed marked antioxidant and free radical scavenging properties, and a significant cytoprotective activity on t-BOOH-treated lymphocytes. Results indicate that C. lumia albedo extract could be exploited as an antioxidant source, suggesting the use of albedo, currently considered a by-product, as a valuable raw material for nutraceutical employments.