During 2019–2020 growing season, stem canker and dieback symptoms were observed in up to 60% of 5-8 year old hawthorn (Crataegus azarolus L.) trees in Mersin province, Turkey. Symptoms appeared as wilting and discoloration of leaves, canker, gummosis, and dieback on stems and branches. The cross-section of symptomatic branches and twigs showed hard brown and V-shaped canker in the wood. Isolation made from surface-sterilized hawthorn twigs (n = 12) on potato dextrose agar (PDA) yielded white or grey-to-dark brown fungal colonies (n = 8) with aerial mycelium and dark pycnidia. Mature conidia were aseptate, ovoid with truncated or rounded base and obtuse apex, and 21.6–25.4 × 7.5–9.8 µm in size (n = 60). These morphological features agreed with those of Diplodia seriata De Not. (Phillips et al. 2007). The internal transcribed spacer (ITS) region, β-tubulin (TUB2) and TEF1-α (tef) genes of the representative isolate (HDs33) were amplified using the primers ITS4/ITS5, Bt2a/Bt2b, and EF1-728F/EF1-986R, respectively (Urbez-Torres et al. 2008), and sequenced (Accession Nos. OL351614 for ITS, OL362020 for TUB2, and OL362021 for tef). BLASTn search of the sequences revealed 100% homology with Diplodia seriata in the NCBI database (MK817042 for ITS, MN318125 for TUB2, and MT120836 for tef). The pathogenicity test was carried out on branches of 4-year-old young hawthorn saplings by inoculating mycelial plugs (5 mm in diameter) cut from a 7-day-old fungal culture into bark wounds on branches of hawthorn saplings (n = 15) in a greenhouse at 30 ℃. Plants treated with sterile agar plugs served as control. After 12 weeks, seven of the fifteen inoculated plants produced canker lesions 24 to 39 mm in length. Control plants remained asymptomatic. The pathogen was consistently reisolated, thus fulfilling Koch’s postulates. D. seriata has previously been reported to cause canker on Crataegus spp. in Canada, Australia, Bulgaria and the United Kingdom (Farr and Rossman 2021). To our knowledge, this is the first report describing D. seriata causing canker and dieback of hawthorn in Turkey.