In the context of modern ruminant nutrition, increasing attention is being directed toward the valorization of agro-industrial by-products as alternative feed ingredients that enhance nutrient utilization efficiency while supporting the sustainability of animal production systems. Milk thistle (Silybum marianum) oilseed cake, a by-product of oil extraction, has emerged as a resource of growing interest due to its favorable nutritional profile and the presence of bioactive compounds with functional properties. This review critically analyzes recent scientific literature addressing the use of milk thistle oilseed cake in ruminant nutrition, highlighting its potential practical relevance as a functional feed ingredient. The available evidence suggests that milk thistle oilseed cake may support inclusion in ruminant diets at moderate levels; however, controlled in vivo studies remain limited, and several proposed mechanisms are inferred from studies on structurally analogous polyphenol-rich by-products rather than from milk thistle cake itself. Further research is needed before precise inclusion recommendations can be established. Special attention is given to the bioactive fraction dominated by the silymarin complex, which may interact with rumen digestive and fermentative processes, influencing nutrient utilization efficiency and oxidative stability. Overall, the findings suggest that milk thistle oilseed cake represents a promising feed resource that aligns with sustainable and efficiency-oriented feeding strategies in modern ruminant production systems.