Long-chain omega-3 polyunsaturated fatty acid (LCn-3PUFA) supplementation may improve symptoms of depression in children and bipolar disorder (BD) in adults. No studies have examined the effectiveness of LCn-3PUFA supplementation in the treatment of mania and depression in juvenile BD (JBD) when given as an adjunct to standard pharmacological treatment. Eighteen children and adolescents with JBD received supplements containing 360 mg per day eicosapentaenoic acid (EPA) and 1560 mg per day docosahexaenoic acid (DHA) for 6 weeks in an open-label study. Intake and fasting red blood cell (RBC) LCn-3PUFA, mania, depression and global function were assessed before and after supplementation. RBC EPA and DHA were significantly higher following supplementation. Clinician ratings of mania and depression were significantly lower and global functioning significantly higher after supplementation. Parent ratings of internalizing and externalizing behaviours were also significantly lower following supplementation. A larger randomized controlled trial appears warranted in this participant population.