Natural products continue to play a significant role in drug discovery and development processes, and many plants have already provided valuable clues for potentially bioactive compounds. Additionally, it has become evident that significant health risks and benefits are associated with dietary food choice. Studies conducted in the past few years have demonstrated that quince (Cydonia oblonga Miller) is a good, safe, and low-cost natural source of different classes of interesting metabolites. In particular, phenolic compounds have application as preventive or therapeutic agents in diseases in which free radicals are involved. Despite the fact that quince fresh or processed fruit properties have attracted most attention, this study focused on antioxidant activities of phenolic compounds extracted from leaves of the Tunisian local variety. Accordingly, chromatographic characterization of methanol extract revealed the existence of nine compounds corresponding to phenolic acids and flavonoids. However, only six compounds could be systematically identified namely, 4-O-caffeoylquinic acid, quercetin-3-O-rutinoside, quercetin-3-O-galactoside, kaempferol-3-O-rutinoside, kaempferol-3-O-glycoside, and kaempferol-3-O-glucoside. Total phenolic compounds and flavonoids showed respectable content as compared to common medicinal plants: 52.5 mg/g dw and 4 mg/g dw, respectively. Consequently, DPPH free radical-scavenging activity and the inhibition of pre-formed radical ABTS.+ by leaf extract were comparable to or even higher than the effect of synthetic antioxidant BHT. In addition, both tests were very highly correlated (r = 0.82) under the effect of quince leaf extract testifying to the sufficiency of only one test to highlight antioxidant potential.