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Simple Summary: The increasing societal concern in animal welfare has changed centuries-old practices in pig breeding. One of the most debatable is the surgical castration of piglets commonly used in many European countries primarily performed to avoid boar taint. Boar taint is an undesirable and unpleasant odor that is released by mature entire male pigs. The two main compounds responsible for boar taint are androstenone and skatole. While androstenone as a steroid is mainly affected genetically, skatole as a product of microbial degradation of the amino acid tryptophan in hind gut can be influenced by nutrition and feeding. Recently, several studies revealed that hydrolysable tannins in the pig diet have the potential to reduce skatole accumulation in fat tissue. Thus, the objective of this study was to determine the effect of tannin administration (1, 2, 3, and 4%—sweet chestnut extract rich in hydrolysable tannins) on skatole as well as androstenone deposition in adipose tissue, and to observe the impact on carcass and meat quality, chemical, amino and fatty acid composition, and boar taint perception. The results showed that tannins in the diet of entire males did not affect chemical composition, androstenone accumulation, amino, and fatty acid profile in subcutaneous tissue. The effect on carcass value was only slight. However, higher doses of tannins (3 and 4%) increased cooking loss, and partially increased (4% dosage) electrical conductivity in semimembranosus muscle. Skatole concentration in fat tissue had a tendency to decline after 2–4% administration compared to control group. Similarly, a tendency to drop in boar taint perception using "hot iron" method was found between control and 2%-supplemented pigs. Higher dietary tannins supplementation (3 and/or 4%) increased several saturated (SFA), monounsaturated (MUFA), and polyunsaturated fatty acids (PUFA) in pork. The slaughtering of entire males increases the probability of incidence of tainted pork due to the presence two main compounds—androstenone and skatole. If a surgical castration of young entire male pigs is stopped in the EU countries, fattening of boars is likely to become one of the most commonly used systems in pig farming. Since skatole production and accumulation in fat tissue can be controlled by dietary approaches, several studies have investigated various feed additives to reduce this compound of boar taint. Ones of the most promising is tannins. The aim of this study was to determine the effect of different dietary tannin level supplementation on carcass, pork quality, chemical, amino and fatty acid composition. as well as perception of boar taint and accumulation of skatole and androstenone in adipose tissue. Eighty entire males were randomly distributed to control (T0) and four experimental groups. Control pigs received standard feed mixture (16.8% CP, 13.9 MJ ME) without any tannin supplementation. Experimental pigs received the same diet with administration of 1% (T1), 2% (T2), 3% (T3) and 4% (T4)—sweet chestnut extract rich in hydrolysable tannins for 40 days (from average live weight of 80 kg until slaughter at average weight 122.28 kg ± 5.63 kg). Dietary tannins supplementation did not show any significant effect on chemical composition, cholesterol content, and amino acid composition of muscle as well as fatty acid composition and androstenone accumulation in adipose tissue. A slight or small effect was observed on carcass and meat quality, respectively. Pigs in groups T4 and/or T3-T4 had higher electrical conductivity in semimembranosus muscle and cooking loss value compared to T1, T2 or T0, T1, and T2 groups (p < 0.05). Tannins in the pig's diet greatly affected fatty acid profile in meat of entire males. The highest tannin levels (4%) increased concentrations of lauric, myristic, vaccenic, linoleic, total PUFA, and n-6 PUFA in muscle compared to the control. Similar results were found in group T3 except for vaccenic, linoleic, and total PUFA. On the contrary, concentrations of heptadecanoic and oleic acids in groups T3 and T4 were lower than those in T1 and T2 groups. Perception of boar taint using „hot iron" method (insertion a hot iron tip of soldering iron into adipose tissue) tended to decrease in T2 group compared with control. Skatole accumulation in fat tissue was reduced in groups T2-T4 at significance level (p = 0.052–0.055) compared to the control pigs. In summary, tannins supplementation had no effect on chemical and amino acid composition as well as fatty acid profile in adipose tissue, and only slight on carcass value. However, 4% concentration of tannins significantly increased content of some fatty acids compared to control group. [ABSTRACT FROM AUTHOR]