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Background & aims: Walnuts are nutrient-dense foods rich in bioactive compounds that may support mental health. However, evidence linking walnut consumption to mood regulation and to changes in urinary metabolites potentially associated with these effects in humans remains limited. This randomized crossover trial (RCT) aimed to evaluate the effects of an 8-week walnut (Juglans regia) intervention on mood states in young adults and to explore the potential mechanistic links through urinary serotonin metabolite (5-hydroxyindoleacetic acid, 5-HIAA), cortisol, and urolithins. Methods: In this 18-week open-label RCT, 31 healthy young adults (mean age 24.0 ± 4.2 years; 90.3% female) consumed 40 g per day of walnuts for 8 weeks (intervention) and followed a nut-free control period of equal length, separated by a 2-week washout. Mood states, perceived stress, well-being, and trait food cravings were assessed using validated questionnaires. Urinary concentrations of 5-HIAA and cortisol were quantified via ELISA and normalized to creatinine. Walnut-derived urolithins were determined in urine samples as biomarkers of intake by HPLC-QTOF-MS. Linear mixed-effects models were used to test differences in outcome variables between intervention and control conditions, and repeated-measures correlations were performed to assess within-individual associations between the outcome variables. Results: Compared to the control condition, the 8-week walnut intervention significantly reduced total mood disturbance scores (-4.85 points, p = 0.030). In addition, the walnut intervention significantly increased urinary 5-HIAA levels, with no changes in urinary cortisol. Increases in urinary 5-HIAA were significantly associated with reductions in fatigue (r = -0.392, p = 0.031) and total mood disturbance (r = -0.362, p = 0.049). Urinary levels of urolithins significantly increased during the intervention and were strongly correlated with concurrent changes in urinary 5-HIAA (r = 0.537 and r = 0.634, respectively). Conclusions: Daily walnut consumption improved total mood disturbance scores in young adults. These findings were accompanied by increased urinary 5-HIAA and urolithin levels. The concurrent changes in these urinary metabolites may reflect dietary, microbial, or peripheral metabolic responses to walnut consumption; however, mechanistic pathways warrant further investigation. These findings highlight the relevance of whole-food strategies in nutritional approaches to mental health.