A new leaf spot of holy basil (Ocimum tenuiflorum) was observed in Kalyani, West Bengal, in September 2022 and 2023. The light to dark brown, circular spots had irregular margins and rarely coalesced to cover the entire leaf surface. The causal pathogen was identified as Colletotrichum siamense through detailed morphological and molecular analyses and caused similar symptoms in pathogenicity tests. This report is the first of C. siamense causing leaf spot on holy basil in India.