Saturated fatty acids induce an inflammatory response in the hypothalamus primarily through TLR4 signaling, contributing to obesity pathogenesis. In animal models, high-fat diets lead to hypothalamic inflammation, causing resistance to anorexigenic hormones like leptin and insulin. This study shows that long-chain saturated fatty acids predominantly activate TLR4, leading to cytokine expression and endoplasmic reticulum (ER) stress. Rats on monounsaturated fat diets do not develop hypothalamic leptin resistance, while TLR4 loss-of-function or pharmacological inhibition prevents diet-induced obesity. TLR4 is a key molecular target for saturated fatty acids in the hypothalamus, triggering inflammatory signaling and resistance to anorexigenic signals. TLR4 activation precedes and determines ER stress induction, with TLR4 being the main receptor for long-chain saturated fatty acids. Inhibition of TLR4 completely blunts ER stress and cytokine expression, while ER stress inhibition alone does not affect TLR signaling. Saturated fatty acids, particularly arachidic acid, induce TLR2 and TLR4 signaling and ER stress in the hypothalamus. TLR4 expression is mainly in microglia, and its loss-of-function protects against obesity and leptin resistance. The study highlights TLR4 as a critical mediator of hypothalamic dysfunction and obesity development, suggesting it as a therapeutic target for obesity-related conditions.Saturated fatty acids induce an inflammatory response in the hypothalamus primarily through TLR4 signaling, contributing to obesity pathogenesis. In animal models, high-fat diets lead to hypothalamic inflammation, causing resistance to anorexigenic hormones like leptin and insulin. This study shows that long-chain saturated fatty acids predominantly activate TLR4, leading to cytokine expression and endoplasmic reticulum (ER) stress. Rats on monounsaturated fat diets do not develop hypothalamic leptin resistance, while TLR4 loss-of-function or pharmacological inhibition prevents diet-induced obesity. TLR4 is a key molecular target for saturated fatty acids in the hypothalamus, triggering inflammatory signaling and resistance to anorexigenic signals. TLR4 activation precedes and determines ER stress induction, with TLR4 being the main receptor for long-chain saturated fatty acids. Inhibition of TLR4 completely blunts ER stress and cytokine expression, while ER stress inhibition alone does not affect TLR signaling. Saturated fatty acids, particularly arachidic acid, induce TLR2 and TLR4 signaling and ER stress in the hypothalamus. TLR4 expression is mainly in microglia, and its loss-of-function protects against obesity and leptin resistance. The study highlights TLR4 as a critical mediator of hypothalamic dysfunction and obesity development, suggesting it as a therapeutic target for obesity-related conditions.