Ongoing biodiversity decline impairs ecosystem processes, including pollination. Flower visitation, an important indicator of pollination services, is influenced by plant species richness. However, the spatio-temporal responses of different pollinator groups to plant species richness have not yet been analyzed experimentally. Here, we used an experimental plant species richness gradient to analyze plant-pollinator interactions with an unprecedented spatio-temporal resolution. We observed four pollinator functional groups (honeybees, bumblebees, solitary bees, and hoverflies) in experimental plots at three different vegetation strata between sunrise and sunset. Visits were modified by plant species richness interacting with time and space. Furthermore, the complementarity of pollinator functional groups in space and time was stronger in species-rich mixtures. We conclude that high plant diversity should ensure stable pollination services, mediated via spatio-temporal niche complementarity in flower visitation. Declining plant diversity has been shown to affect ecosystem processes such as plant-pollinator interactions, but it is currently not known if or how spatio-temporal niche partitioning of pollinators is affected by plant biodiversity. In a grassland biodiversity experiment, where we manipulated plant species richness and studied spatio-temporal resource use of pollinators, we found that complementarity of pollinator groups in space and time was stronger in species-rich mixtures. Our study shows that high plant diversity may ensure stable pollination services due to increased pollinator complementarity.