The ambrosia beetle Xyleborus monographus (Fabricius) inhabits oak wood (Quercus sp.). These insects carry fungal inoculum in specialised structures called mycangia. It is assumed that, apart from their symbiotic fungi, they may also transport other fungi, including those potentially responsible for blue stain or wood decay. The composition and diversity of fungi isolated from the mycangia and bodies of X. monographus beetles were examined at two locations in Poland: the Krotoszyn FD (Forest District) and the Nowa Sól FD. The fungal colonies obtained were identified based on genetic sequences and morphology. In total, 143 isolates were obtained, representing 20 taxa from 16 fungal genera. The most frequently isolated genus was Penicillium (95% of samples), while Talaromyces, Mucor, Yamadazyma, and Trichoderma were found in 20–30% of the individuals examined. Fungi potentially associated with blue stain and wood decay were also detected: Graphium basitruncatum, Sporothrix pseudoabietina, Clonostachys rosea, Fusarium solani, and Diaporthe sp. These fungi are known components of the wood microbiome, capable of causing discolouration and degradation of sapwood, which may lead to economic losses in oak timber. The results suggest that X. monographus may serve as a vector for saprotrophic and potentially blue-stain fungi, contributing to the deterioration of oak wood quality. The data provide a basis for further research on the role of ambrosia beetles in the dissemination of staining and wood-decaying fungi in hardwoods, as well as for developing methods to reduce economic losses caused by wood discolouration and degradation.