Decreased n-3 fatty acid levels have been reported in patients with depression, schizophrenia or Alzheimer's disease. Recently, eicosapentaenoic acid (EPA) has been used to treat several psychiatric and neurodegenerative diseases due to its anti-inflammatory and neuroprotective effects. A total of six out of seven clinical trials have shown that EPA significantly improved depressive symptoms when compared with the placebo-treated populations. Several investigations have also reported that EPA could effectively treat schizophrenia. A case report and a clinical trial have shown that EPA was beneficial for the management of most symptoms of Huntington's disease, while a more extensive clinical investigation has demonstrated that EPA could only improve motor functions. Further clinical studies are required to fully explore the effects of EPA on other neurodegenerative diseases. The limitations of previous studies and further research directions have also been discussed.