Currently, consumers are increasingly interested in obtaining high-quality and healthy lamb meat. Compared to grain-based diets, dietary forage legumes such as alfalfa and condensed tannin (CT)-rich sainfoin increase the levels of polyunsaturated fatty acids (PUFAs) that are beneficial for health in lamb meat thanks to their high content in PUFA and/or their impact on ruminal biohydrogenation. However, they can therefore adversely affect its oxidative stability. Thus, the impact of dietary forage legumes on lamb longissimus thoracis (LT) muscle FA composition and their stability to peroxidation was studied in 36 Romane lambs grazing alfalfa (AF; n = 12) or alfalfa plus daily supplementation with CT-rich sainfoin pellets (AS; n = 12; 15 g DM/kg BW, 42 g CT/kg DM) or stall-fed concentrate and grass hay indoors (SI; n = 12). Lambs were slaughtered at a mean age of 162 ± 8.0 days after an average experimental period of 101 ± 8.1 days. Forage legumes-grazing lambs outperformed SI lambs in LT nutritional quality, with more conjugated linoleic acids and n-3 PUFAs, especially 18:3n-3, eicosapentaenoic and docosahexaenoic acids (P < 0.001), and thus lower n-6 PUFA/n-3 PUFA and 18:2 n-6/18:3 n-3 ratios (P < 0.001). Peroxidizability index was higher (P < 0.001) in LT muscle of forage legumes-grazing lambs. Concurrently, two endogenous antioxidant enzyme activities, superoxide dismutase and glutathione peroxidase, were, respectively, similar and lower (P < 0.001) for forage legumes-grazing compared with SI lambs. A lower vitamin E level in SI lambs compared with forage legumes-grazing lambs (1.0 v. 3.8 mg/g, P < 0.001) could explain that malondialdehyde content, a marker of lipid oxidation intensity, was 0.63 µg/g in SI after 8 days in aerobic packaging conditions, whereas it remaining steady at 0.16 µg/g in forage legumes-grazing lambs. Dietary forage alfalfa thus improved FA composition of lamb LT muscle and their stability to oxidation when compared to SI lambs. However, supplementation of alfalfa-grazing lambs with CT-rich sainfoin pellets did not affect the nutritional quality of LT muscle FAs.