Understanding mechanisms underlying the origin and maintenance of biodiversity is a central goal of modern ecological and evolutionary research. Ecologists have recognized the potentially important role of temporal niche in promoting species coexistence and diversity, yet little is known about how temporal niche affects the evolution of biodiversity. Here we show that temporal niche strongly influences biodiversity dynamics in rapidly evolving bacteria. An ancestral bacterium quickly diversifies when provided with constant spatial niche opportunities or when experiencing temporal niche dynamics. However, only in communities with temporal niches, which promote frequency-dependent selection and the positive growth of new mutants, is the accumulated phenotypic diversity able to persist. Overall, the presence of temporal niche opportunities eliminates the overshooting dynamics of adaptive radiation typically seen in this and other systems. These results suggest that temporal niche may have an essential role in the maintenance of biodiversity over evolutionary time.