Abstract The physicochemical quality parameters of meat from finishing lambs supplemented with quebracho tannin extract (QTE) were evaluated. Lambs were assigned to four treatment groups: CON—animals fed basal diet (BD); QTE 1% (T1%)—BD + tannins at 1% dry matter intake (DMI); QTE 3% (T3%)—BD + tannins at 3% DMI; and QTE 6% (T6%)—BD + tannins at 6% DMI. Longissimus thoracis et lumborum samples were vacuum packaged and stored at 4 °C for 28 days. The samples were evaluated weekly for proximate composition, pH, color, myoglobin forms, cooking loss, shear force, lipid (thiobarbituric acid reactive substances—TBARS), and protein (carbonyl and thiol) oxidation. Chemical composition and pH did not vary among treatments. The meat from lambs fed with higher proportion of QTE (T6%) presented lower redness, chroma, and oxymyoglobin content, and higher yellowness, hue angle, cooking loss, shear force, TBARS, and carbonyl content than the meat from the other groups. Our findings indicate that higher dose of QTE may exert pro-oxidative effects in lamb meat, negatively affecting its quality.