Depression and dementia are major public health problems. Major depressive disorder (MDD) and Alzheimer's disease (AD) reciprocally elevate the risk for one another. No effective drug is available to treat AD and about one-third of depressive patients show treatment resistance. The biological connection between MDD and AD is still unclear. Uncovering this link might open novel ways of treatment and prevention to improve patient healthcare. Here, we discuss recent studies specifically on the role of human adult hippocampal neurogenesis (AHN) in MDD and AD. We compare diverse approaches to analyse the effect of MDD and AD on human AHN and analyse different studies implicating the role of human AHN as a potential converging mechanism in MDD and AD.