The olfactory neuroepithelium undergoes continual neurogenesis and, after extensive lesions, fully regenerates to maintain sensory function. The stem cell population underlying this regenerative capacity remains elusive. Here we show that mouse horizontal basal cells (HBCs) function as adult olfactory neuroepithelium neural stem cells and examine their distinct dynamics in olfactory neuroepithelium maintenance and regeneration. Fate-mapping analysis after olfactory neuroepithelium lesioning shows that HBCs are competent to regenerate both neuronal and non-neuronal olfactory neuroepithelium lineages. HBCs serve as a reservoir of long-lived progenitors that remain largely quiescent during normal neuronal turnover or even after acute, selective loss of mature neurons. Under these conditions, previously identified progenitors are largely responsible for tissue maintenance. Yet after extensive injuries that deplete resident neuronal precursors, HBCs transiently proliferate and their progeny fully reconstitute the neuroepithelium. Our data support a new model of adult neurogenesis in which distinct cell populations mediate normal neuronal turnover and neuronal replacement upon traumatic injury.