Mushroom-forming fungi are important sources of food and medicine in many regions of the world, and their development and health are known to depend on various microbes. Recent studies have examined the structure of mushroom-inhabiting bacterial (MIB) communities and their association with local environmental variables, but global-scale diversity and determinants of these communities remain poorly understood. Here we examined the MIB global diversity and community composition in relation to climate, soil and host factors. We found a core global mushroom microbiome, accounting for 30% of sequence reads, while comprising a few bacterial genera such as Halomonas, Serratia, Bacillus, Cutibacterium, Bradyrhizobium and Burkholderia. Our analysis further revealed an important role of host phylogeny in shaping the communities of MIB, whereas the effects of climate and soil factors remained negligible. The results suggest that the communities of MIB and free-living bacteria are structured by contrasting community assembly processes and that fungal-bacterial interactions are an important determinant of MIB community structure.