During a survey conducted in Hamedan Province, Iran, Thompson seedless grapevine (Vitis vinifera) bunches were observed exhibiting symptoms of brown ripe rot. To identify the causal agent, samples were collected, and fragments from the infected tissues were cultured on PCA and PDA media. Morphological studies and sequencing analyses of the ITS region were performed to identify the emerging fungus. To confirm the pathogenicity of the fungal isolate, Koch’s postulates were applied. Based on morphological assessments, pathogenicity test, and the ITS sequences, the causal agent was identified as Arthrinium sacchari (Apiosporaceae, Sordariomycetes). This study provides new information on ripe rot caused by A. sacchari in grapevine plants.