Abstract Long-chain n -3 PUFA (LC n -3 PUFA) prevent, in rodents, insulin resistance (IR) induced by a high-fat and/or fructose diet but not IR induced by glucocorticoids. In humans, contrasting effects have also been reported. We investigated their effects on insulin sensitivity, feed intake (FI) and body weight gain in genetically insulin resistant male obese (fa/fa) Zucker (ZO) rats during the development of obesity. ZO rats were fed a diet supplemented with 7 % fish oil (FO) + 1 % corn oil (CO) (wt/wt) (ZO FO ), while the control group was fed a diet containing 8 % fat from CO (wt/wt) (ZO CO ). Male lean Zucker (ZL) rats fed either FO (ZL FO ) or CO (ZL CO ) diet were used as controls. FO was a marine-derived TAG oil containing EPA 90 mg/g + DHA 430 mg/g. During an oral glucose tolerance test, glucose tolerance remained unaltered by FO while insulin response was reduced in ZO FO only. Liver insulin sensitivity (euglycaemic–hyperinsulinaemic clamp + 2 deoxyglucose) was improved in ZO FO rats, linked to changes in phosphoenolpyruvate carboxykinase expression, activity and glucose-6-phosphatase activity. FI in response to intra-carotid insulin/glucose infusion was decreased similarly in ZO FO and ZO CO . Hypothalamic ceramides levels were lower in ZO FO than in ZO CO . Our study demonstrates that LC n -3 PUFA can minimise weight gain, possibly by alleviating hypothalamic lipotoxicity, and liver IR in genetically obese Zucker rats.