Common hazel (Corylus avellana) is an economically valuable nut crop worldwide. There has been a rapid increase in a number of hazelnut growing countries as well as in the acreage of cultivated hazelnuts within the major producing countries including in Iran. Even if, hazelnuts are relatively undemanding and adaptable to many different ecological conditions, hazelnut is susceptible to a large number of diseases such as bacterial blight (caused by Xanthomonas arboricola pv. corylina), powdery mildew disease (caused by Phyllactinia guttata), Mamianiella leaf spot disease (Mamianiella coryli) and very recently trunk diseases, which threaten hazelnut trees productivity and longevity in Iran. Phyllactinia guttata is a widespread powdery mildew species on this host in many hazelnut-producing countries including Iran. Very little is known on diseases and disorders of hazelnut trees in Iran. In this study we report a new powdery mildew species on common hazelnut in Iran. During excursions to the forest areas in the East Azerbaijan province and Ardabil province in August 2017, heavily infected hazelnut trees with powdery mildew disease were observed. Samples were collected from symptomatic leaves and fruit clusters in paper bags and taken to the laboratory. Sexual and asexual fungal structures including chasmothecia and hyphal mass were mounted in distilled water and examined using light microscopy. Sequence data of the ITS-rDNA region were analyzed to further confirm the morphological identification. Fungus is identified E. corylacearum based on morphological and molecular characteristics. To the best of our knowledge, this study provides the first occurrence of E. corylacearum on C. avellana in Iran. Considering severe incidence of powdery mildew caused by E. corylacearum on common hazelnut in the sampling sites in this study, it is necessary to study different aspects of the disease including pathogen distribution, biology, diseases cycle and management.