Dietary inclusion of grape pomace (GP) for finishing lambs is expected to improve performance and product quality, thanks to its rich polyphenolic content and favourable fatty acid (FA) profile. This study determines the implications of feeding varying levels of GP (Vitis vinifera L. cultivar SiahGohar) on production performance, rumen fermentation, blood parameters, and meat FA composition in finishing lambs. Twenty-one Baluchi lambs [28.35 ± 1.54 kg of initial body weight (IBW), mean ± standard deviation (SD)] were randomly assigned to four diets containing 0, 6%, 12%, and 18% g GP [dry matter (DM) basis] in total mixed rations (TMR). The trial last for 90 days, preceded by 21 days of adaptation. Results showed that the dietary inclusion of GP did not affect the DMI (p = 0.8452) of fattening lambs, while it improved the final body weight (FBW, p = 0.027), average daily gain (ADG, p = 0.016), and feed to gain ratio (F:G, p = 0.001). Lambs fed on GP diets tended to increase ruminal pH (p = 0.100); however, NH-N levels remained unaffected (p = 0.160). There was no diet effect (p > 0.5) on plasma concentrations of glucose, triglyceride, cholesterol, albumin, creatinine, BUN, aspartate aminotransferase (AST), and alanine aminotransferase (ALT). Dietary inclusion of GP also led to increased muscle accumulation of C18:2 n-6 (p < 0.0001), C18:3 n-3 (p = 0.005), C20:3 n-3 (p < 0.0001), total MUFA (p < 0.0001), and PUFA (p < 0.0001). GP also mitigated lipid oxidation in lamb's meat, as indicated by lower (p < 0.0001) malondialdehyde (MDA) levels. Further investigation is warranted to verify the potential effects of GP on the rumen microbiome, colour stability, and shelf-life of meat, as well as to elucidate the mechanisms underlying the action of polyphenols and their composition contained in GP.