Mesenchymal stem/stromal cells (MSCs) are promising candidates in regenerative medicine, but their effectiveness is significantly influenced by the surrounding metabolic and nutritional conditions. Increasing evidence suggests that lipids act not only as energy sources but also as regulators of MSC fate. This review explores how lipid metabolism influences the balance among stemness, immunomodulation, and differentiation into adipogenic or osteogenic lineages. It does so through mechanisms such as fatty acid uptake, β-oxidation, de novo lipogenesis, and membrane remodeling, all orchestrated by CD36, carnitine palmitoyltransferase 1A, PPARγ, AMP-activated protein kinase, and the PI3K/AKT/mTOR pathway. We then examine how diet reshapes the MSC lipidome: obesity and high-fat diets promote adipogenesis and senescence, while omega-3 fatty acids, caloric restriction, micronutrients, and a balanced microbiota help preserve regenerative capacity. Lastly, we discuss how combining lipidomics with multi-omics could uncover lipid-metabolic signatures and regulatory nodes that connect diet to MSC function. Overall, the diet–lipid–MSC axis emerges as a modifiable determinant of MSC function.