Electroconvulsive therapy (ECT) is an effective treatment for major depressive episodes; however, the neural mechanisms underlying its therapeutic effects remain largely unknown. This review summarizes the findings of functional neuroimaging studies on the effects of the ECT treatment in patients with unipolar depression. Functional neuroimaging studies show that ECT induces changes in cerebral metabolism, blood flow, neurotransmitter activity, neuronal metabolites, and brain functional connectivity. Nevertheless, most of these studies are cross-sectional, with small sample sizes, and the findings across these studies are inconsistent and often contradictory. The levels of brain functional changes are seldom correlated with alleviation of depressive symptoms. Based on the results of current functional neuroimaging studies, one cannot draw any conclusions about the exact neural mechanisms of the antidepressant effect of ECT. Additional multicenter, large-sample, unified-protocol studies with long-term follow-up and multiple observational markers are needed to clarify and cross-validate the neural mechanisms of the antidepressant effect of ECT and to explore validated neuroimaging markers for predicting and evaluating the clinical efficacy of ECT treatment.