Silymarin is the flavonoid extracted from Silybum marianum seed. It has anti-inflammatory and antibacterial properties, and it supports liver health. The growth performance parameters, nutrient digestibility, and blood profile in growing pigs fed the dietary Silybum marianum seed extract were analyzed in this study. A total of 196 crossbred growing pigs ((Yorkshire × Landrace) × Duroc) were allocated into three dietary groups: CON: basal diet; TRT1: basal diet with 0.05% Silybum marianum seed extract; and TRT2: basal diet with 0.10% Silybum marianum seed extract. Results showed that pigs fed with Silybum marianum seed extract up to 0.10% increased average daily gain (p < 0.10) and feed intake (p < 0.05). The digestibility of dry matter, nitrogen, and energy were increased linearly (p < 0.05) with addition of Silybum marianum seed extract up to 0.10%. However, serum bile acids, alanine aminotransferase (ALT), and aspertate aminotransferase (AST) were not changed (p < 0.05). In conclusion, the significant improvements in average daily gain, feed intake, and nutrient digestibility indicate that Silybum marianum seed extract can positively influence the growth of growing pigs. Finally, natural feed additives like Silybum marianum seed extract may be used as an efficacious growth promoter and ultimately contributing to sustainable pig farming.