Background: There is still limited knowledge about the mechanism of action of electroconvulsive therapy (ECT) in the treatment of depression. Substantial evidence suggests a role for the immune-moderated tryptophan (TRP)-kynurenine (KYN) pathway in depression; i.e. a depression-associated disturbance in the balance between the TRP-KYN metabolites towards a neurotoxic process. We, therefore, aimed to investigate the impact of ECT treatment on the TRP-KYN pathway, in association with ECT-related alterations in depressive symptoms. Method: Twenty-three patients with unipolar or bipolar depression, treated with bilateral ECT twice a week were recruited. Blood serum samples, and depression scores using the Hamilton Depression Rating Scale-17 items (HDRS) as well as the Beck Depression Inventory (BDI) were collected repeatedly during the period of ECT and until 6 weeks after the last ECT session. TRP and KYN metabolites were analyzed in serum using the High Performance Liquid Chromatography. Four patients could not complete the study; thereby yielding data of 19 patients. Analyses were performed using multilevel linear regression analysis. Results: There was an increase in kynurenic acid (KYNA) (B = 0.04, p = 0.001), KYN/TRP ratio (B = 0.14, p = 0.001), KYNA/KYN ratio (B = 0.07, p < 0.0001), and KYNA/3-hydroxykynurenine ratio (B = 0.01, p = 0.008) over time during the study period. KYN (B = -0.02, p = 0.003) and KYN/TRP (B = -0.19, p = 0.003) were negatively associated with total HDRS over time. Baseline TRP metabolite concentrations did not predict time to ECT response. Conclusion: Our findings show that ECT influences the TRP-KYN pathway, with a shift in TRP-KYN metabolites balance towards molecules with neuroprotective properties correlating with antidepressant effects of ECT; thereby providing a first line of evidence that the mechanism of action of ECT is (co)mediated by the TRP-KYN pathway.