Endophytic fungi are increasingly being considered for their ecological role and effects on plant protection and nutrition. Their contribution in improving fitness can be particularly relevant in the case of semi-extensive tree crops, such as hazelnut. In this manuscript, the occurrence and properties of the species Arcopilus aureus are reviewed, following its finding as a component of the endophytic microbiome of hazelnut in the distant geographic and climatic contexts of Poland and Italy, representing an indication of its widespread association with this plant and a possible role in protection against biotic adversities.