Depression, as a neuropsychiatric disorder with high prevalence, strong invisibility and high relapse rate, is a serious threat to human physical and mental health. The hippocampus, a key brain region for regulating emotional and cognitive functions, has been found to have persistent neurogenesis in its dentate gyrus during adulthood. In recent years, a large body of evidence suggests that adult hippocampal neurogenesis processes are closely related to the pathophysiology of depression. However, the causal relationship between adult hippocampal neurogenesis and the pathology of depression has not been systematically summarized. In this review, we analyzed four aspects of evidence supporting the ‘adult hippocampal neurogenesis hypothesis’ of depression: reduction of adult hippocampal neurogenesis induces depressive phenotypes and related pathologies; depression models reduce adult hippocampal neurogenesis; increased adult hippocampal neurogenesis exerts antidepressant effect; and current antidepressants treatment increases adult hippocampal neurogenesis. This review is the first to comprehensively summarize the causal role of adult hippocampal neurogenesis in depression, providing new perspectives on the mechanisms of depression and antidepressants.