This study aimed to formulate a diet for finishing lambs that included chestnut shells, an underexploited by-product of chestnut industry, and evaluate its effects on in vivo performance and meat quality. Twenty-eight male lambs (race Romane; 27.9 ± 2.7 kg bodyweight) were divided into 4 groups and fed 4 different pelleted diets: one control, one containing chestnut shells (CNS), one containing sainfoin, and one containing both. After 21 days of feeding trial, at slaughter, rumen and abomasum digesta were sampled for the analysis of fatty acid (FA) profile, and meat was analysed for FA profile, vitamins content, and oxidative stability. All lambs showed similar growth performance and carcass characteristics. The CNS diet limited (P = 0.001) ruminal biohydrogenation, increasing (P < 0.050) the C18:1 trans11 proportion in both rumen and abomasum. Consequently, the C18:1 trans11 content of CNS meat was more than 50% higher (P = 0.006) than in the other groups. No differences in the discolouration and lipid oxidation of raw meat were observed over 9 days of refrigerated storage. The phenolic compounds of chestnut shells may have preserved the low native α-tocopherol level of the CNS diet. Chestnut shells can be fed to lambs without detrimental effect on performance, potentially improving meat FA profile.