Hoplitis robusta (Nylander) is a rare and poorly known osmiine bee species occurring in the subalpine zone of the Alps. The discovery of two nests of H. robusta in a thin branch of a dead fallen spruce on a sunny clearing of a subalpine spruce forest allowed the investigation of the nest architecture, the analysis of the larval diet and the assessment of the nest building material. X-raying, computed tomography and subsequent dissection of the nest branch revealed that the nests were built in L-shaped pupation tunnels of cerambycid beetles, which were probably cleaned from wood debris by the female bees with the aid of their large and powerful mandibles after nest site selection. The two nests contained five and six linearly arranged brood cells separated from each other by thin partitions built from masticated green leaves (“leaf pulp”). They were sealed at their opening by a thick plug consisting of several successive layers of leaf pulp constructed immediately behind each other. Microscopical analysis of the larval provisions of eight brood cells and of 41 pollen loads of females from museum and private collections showed that H. robusta exhibits a strong preference for the pollen of Potentilla (Rosaceae). Based on field observations, DNA metabarcoding of one nest plug and stereomicroscopic analysis of the leaf pulp matrix, Potentilla was also identified as an important source for the leaf pulp needed for nest construction, rendering H. robusta one of the few bee species known to collect floral resources and nest building material from the very same plant.