Objective The present investigation was intended to test the hypothesis that the elderly provided with the frequent consumption of fishes marinated in essential oil of Perilla frutescens (EOPF) or not would experience fewer depressive symptoms after 6 months. Methods A total of 180 participants were recruited from Sina Hospital, Mashhad, Iran, who were diagnosed with depression based on Diagnostic and Statistical Manual of Mental Disorders, 4th edition, Text Revision and Beck Depression Inventory. Participants (n = 180) were randomly assigned in a 1:1:1 ratio to Groups A, B, and C. The last two were provided with an instruction to consume Caspian white fish marinated in the presence or absence of EOPF (434 g each week or four meals per week). Group A served as the control with the common diet. The outcome measures were performed using the Beck Depression Inventory and the General Health Questionnaire. Results There were no statistically significant differences in depressive symptom scores between groups with frequent fish consumption as compared with the control (p > 0.05). Yet adjustment for covariates showed that there was a significant reduction in depression among them (p < 0.05). Moreover, consumption of fish and EOPF was associated with more considerable improvements than Groups A and B (p < 0.05). Conclusions It could be concluded that high intakes of unsaturated fatty acids can afford to diminish likelihood of late‐life depression. Copyright © 2017 John Wiley & Sons, Ltd.