The aim of this study was to determine the effect of milk thistle seed supplementation in pig fattener diets on productive traits, slaughter carcass traits, and meat quality as well as meat, backfat and liver fatty acid composition. The experiment was carried out using 60 barrows, with an initial body weight of 25.0 ± 0.5 kg. The experiment lasted for 100 days. The animals were divided into 3 groups, with 20 pigs in each. The diet of the control group (group I) did not have thistle supplementation, while the remaining groups received 3% (group II) or 6% (group III) thistle in the mixture. Analysis showed that the milk thistle seeds contained relatively high contents of protein, fat, lysine, linoleic acid, oleic acid, and active flavonolignans – specifically silybin A and B (26.4 g), isosilybin (5.3 g), silychristin (10.6 g) and a low amount of silydianin (0.1 g). The 3% milk thistle seed addition improved the average daily weight gains by about 2% and 6% addition improved the gains by 3.8% during the grower period. There was no significant effect of milk thistle seeds on carcass meat content but the backfat layer was thinner. The meat of pigs which received milk thistle was more saturated in red (significantly higher a* values) and less saturated in yellow (significantly lower b* values) for both fresh and cooked samples and its texture, water-holding capacity and oxidative capacity were all improved (thiobarbituric acid reactive substances (TBARS) index lower by more than 30%). The addition of milk thistle seeds decreased the cholesterol content in lumborum muscle, backfat and liver, and these tissues were also characterised by higher polyunsaturated fat (PUFA) content and lower PUFA n-6 : n-3 ratio. Milk thistle seeds increased the hypocholesterolemic : hypercholesterolemic (h : H) index in all tissues and reduced the thrombogenic (TI) and atherogenic (AI) indices. Summing up the results, milk thistle seeds at supplementation levels of 3 and 6% are indicated to be a useful feed admixture for fattened pigs to improve meat quality and oxidative stability. The higher dose of milk thistle seeds (6%) appeared to be more effective in improving weight gains, feed utilisation, PUFA content in tissues, and water-holding and antioxidant capacities.