Fungal endophyte communities are mainly driven by host plant identity and geographic location. However, little is known about interactions between endophytes and characteristics of the host plant such as leaf functional traits, which vary both among and within host species. Previous studies focused on a limited number of host plant species and did not control for varying conditions in the host's neighborhood, which affect leaf functional traits and, in turn, might affect fungal endophyte communities. Using a tree diversity experiment in which all trees grow under standardized conditions, we were able to assess the contributions of host tree identity, host neighborhood species richness, and host community composition as well as the variation of leaf traits caused by these factors on taxonomic richness and community composition of foliar fungal endophytes. We used next-generation amplicon sequencing to analyze the fungal endophyte community and visible–near infrared spectrometry data to predict the mean values and the intra-individual variation of leaf traits in individual trees. We found both mean trait values and intra-individual trait variation to have significant effects on endophyte richness. Mean trait values of leaf dry matter content, leaf carbon, leaf nitrogen, and leaf carbon-to-nitrogen ratio exhibited negative effects on endophyte richness, whereas specific leaf area and leaf phosphorus content increased endophyte richness. Additionally, intra-individual leaf-trait variation generally had positive effects on richness. Overall endophyte community composition was influenced by mean leaf dry matter content and specific leaf area. Ascomycota were influenced by the specific leaf area, whereas Basidiomycota responded to leaf dry matter content. We demonstrate that functional leaf traits affect foliar endophyte communities, with positive diversity effects of host leaf nutrients that are essential, and likely limiting, for fungal endophytes. Although our study emphasizes the role of leaf traits in shaping fungal communities, we also acknowledge that these dynamic interactions could lead to traits being influenced by microbes through microbe–plant interactions.