Cells store energy in the form of neutral lipids packaged into micrometer-sized organelles named lipid droplets (LD). These structures emerge from the endoplasmic reticulum (ER), but their biogenesis remains poorly understood. Using molecular simulations, we found that fat accumulation and LD formation are described by a liquid-liquid phase separation (LLPS) process. Within this framework, we could identify how ER membrane properties modulate LD formation, and we could directly test our computational predictions by combining yeast genetics with fluorescence microscopy. Our data suggest that the specific lipid composition of the ER together with its peculiar physical properties, such as low membrane tension and membrane curvature, promote the packaging of neutral lipids into LD, preventing their accumulation in the ER membrane. Our results provide a new conceptual understanding of LD biogenesis in the context of ER homeostasis and function.