The widespread ban on in-feed antibiotics in many regions has driven the search for natural alternatives to maintain health and production efficiency in swine and poultry. Phytogenic feed additives (PFAs) derived from herbs and plant extracts have emerged as promising candidates owing to their antioxidant, anti-inflammatory, and antimicrobial properties. Among these, silymarin-a flavonolignan complex extracted from milk thistle (Silybum marianum)-has attracted particular attention due to its hepatoprotective and growth-promoting activities. This review summarizes the chemical characteristics and mechanisms of action of silymarin/silybin. Also, evidence from both experimental and field studies shows that silymarin improves growth performance, nutrient digestibility, gut health, and reproductive outcomes. Advances in formulation technologies, such as micellization, have been addressed for improved bioavailability of silymarin. Despite these promising results, further long-term field studies and economic evaluations are needed to fully integrate silymarin into commercial animal production systems.