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Unsatisfactory responses to major depressive disorder (MDD) therapeutics available necessitated up-to-date treatment approaches. This study sought to investigate the efficacy and tolerability of adjunctive l -theanine, a green tea constituent with neuropsychotropic effects, for MDD. Sixty MDD (DSM-5) patients were equally assigned to receive sertraline (100 mg/d) plus either l -theanine (200 mg/d) or matched placebo in a six-week randomized, parallel-group, double-blind, placebo-controlled study. The participants were assessed using the Hamilton depression rating scale (HDRS) at baseline and weeks 2, 4, and 6. Changes in scores, early improvement, response and remission rates, and adverse events were compared between the groups. Twenty-five participants in each group, a total of 50 patients, completed the study. All baseline characteristics were similar between the groups. The general linear model repeated-measures analysis demonstrated a significant time-treatment interaction effect for HDRS during the trial (p -value = 0.014), indicating more remarkable symptom improvement in the l -theanine group. A greater reduction in HDRS scores was observed in the l -theanine group from baseline to weeks 2, 4, and 6 (p -values = 0.02, 0.03, and 0.01, respectively). All patients responded to sertraline plus l -theanine until week 6. l -theanine was superior to placebo regarding response to treatment and remission rates at week 6 (p -values = 0.05 and 0.02, respectively). The frequency of side effects was comparable between the groups. The small sample size and short study period were the limitations. l -theanine adjunct to sertraline outperforms placebo in treating MDD in a safe manner. Further long-term, large-scale studies are recommended to confirm this evidence. • l -theanine is an amino acid found naturally in green tea, black tea, and mushrooms. • It is beneficial and tolerable in depression, obtained from a gold-standard design. • l -theanine has been reported to modulate GABA, serotonin, and dopamine levels. • It may also exert effects through the glutamatergic and neuroimmune mechanisms. [ABSTRACT FROM AUTHOR]