Background <p>The growing evidence of the bidirectional relationship between gut microbial flora and brain neurochemistry and consequently behavior, has recently garnered significant attention as the gut-brain microbial axis. Considering the conflicting results of trials investigating the effects of probiotic therapy on depression, the present study aims to evaluate the impact of <i>Lactobacillus casei</i> on postpartum depression.</p> Method <p>This randomized controlled trial was conducted on 74 postpartum women admitted to Al-Zahra Hospital, Tabriz, Iran. Eligible women were randomly assigned into the intervention and placebo groups using block randomization method. The intervention group received 500&#xa0;mg of capsules containing <i>Lactobacillus casei</i> 431 at a dose of 1.5 × 10⁹ cfu/ml once daily, starting the day after delivery and continuing until 21 ± 1 days postpartum. Depression levels were assessed at 5 ± 1, 15 ± 1, and 21 ± 1 days after delivery using independent t-tests and RMANOVA.</p> Results <p>The mean depression score (SD) at 5 ± 1 days postpartum was 4.7 (2.5) in the probiotic group and 5.6 (2.4) in the placebo group. By 15 ± 1 days postpartum, the scores increased to 9.28 (1.68) in the probiotic group and 11.57 (2.55) in the placebo group. At 21 ± 1 days postpartum, the mean depression scores were 7.39 (2.96) and 8.11 (3.61) in the probiotic and placebo groups, respectively. No significant difference was observed between the groups (Mean Difference: -1.68, 95% CI: 0.08 to -2.56 to, <i>p</i> = 0.71).</p> Conclusion <p><i>Lactobacillus casei</i> had no significant effect on alleviating postpartum depression symptoms. Further studies should focus on starting probiotic in pregnant women for a longer duration of use.</p> Clinical trial number <p>Iranian Registery of Clinical Trials-Beta vertion, <a href="https://irct.behdasht.gov.ir/trial/71775">https://irct.behdasht.gov.ir/trial/71775</a> (IRCT20170506033834N7), registered 2021-08-11.</p>