Background There is some evidence in the literature that emphasising fish consumption may assist with weight loss. The aim was to assess the effects of advice to consume 2 fish meals per week in a weight loss diet. Methods A parallel randomised placebo-controlled trial was conducted in 118 obese Australian adults (mean BMI +/- SD 31.3 +/- 3.5 kg/m2; mean age +/- SD 45 +/- 10 y; 28% male). Participants received low calorie dietary advice + placebo (1 g olive oil; Control), low calorie dietary advice emphasising fish + placebo (Fish), or low calorie dietary advice emphasising fish diet + LCn3PUFA supplements (Fish + S). Individualised advice targeted 2 MJ energy deficit (30%E fat, 45%E carbohydrate and 25%E protein) with or without two servings (180 g) fatty fish/wk. Results All groups lost weight at 12 months (Control -4.5 kg vs. Fish -4.3 kg vs. Fish + S -3.3 kg; p < 0.001) and percentage body fat (Control: -1.5% vs. Fish: -1.4% vs. Fish + S: -0.7%; p < 0.001) but there were no significant differences between groups. Cardiovascular disease risk factors changed as expected from weight loss. Conclusions Advice to consume 2 fish meals per week did not enhance the effects on weight loss of a healthy low calorie diet.