BACKGROUND: This study assessed whether a combined intervention of omega-3 polyunsaturated fatty acids (PUFAs) and psychoeducation better improved mild to moderate depression in workers compared to psychoeducation alone. METHODS: This study was a double-blinded, parallel group, randomized controlled trial that compared the intervention group, receiving omega-3 fatty acids, with a control group, receiving a placebo supplement. Participants receiving omega-3 fatty acids took 15 × 300 mg capsules per day for 12 weeks. The total daily dose of omega-3 PUFAs was 500 mg docosahexaenoic acid and 1000 mg eicosapentaenoic acid (EPA). The Beck Depression Inventory®-II (BDI-II) was used to assess the severity of depression after treatment. RESULTS: After 12 weeks of treatment, BDI-II scores were significantly lower in the placebo and omega-3 group, when compared to their respective baseline scores (Placebo: t = - 4.6, p < 0.01; Omega-3: t = - 7.3, p < 0.01). However, after 12 weeks of treatment, we found no significant difference between both groups with respect to changes in the BDI-II scores (0.7; 95% CI, - 0.7 to 2.1; p = 0.30). LIMITATIONS: This study did not measure blood omega-3 fatty acid concentration and presented a high-dropout rate. Moreover, our results may not be generalizable to other regions. CONCLUSIONS: The results show that a combination of omega-3 fatty acids and psychoeducation and psychoeducation alone can contribute to an improvement in symptoms in people with mild to moderate depression. However, there is no difference between the interventions in ameliorating symptoms of depression.