The economic importance of Cannabis sativa is gaining attention around the world due to its various uses in agriculture, textiles, bio-composite, paper-making, automotive, construction, bio-fuel, functional food, oil, cosmetics, personal care, and pharmaceutical industry. Accurate identification of leaf diseases related to cannabis cultivation is very important as many products from cannabis are related to the leaves. During a survey of cannabis cultivations in Chiang Rai province in Thailand, cannabis plants showing olive leaf spot symptoms were observed. Morphological studies indicated that it belongs to the genus Pseudocercospora. Many Pseudocercospora species are important plant pathogens causing many different types of plant diseases. We conducted both morphological and multilocus phylogeny with the internal transcribed spacer (ITS1, 5.8S, ITS2), the partial actin (act), translation elongation factor 1-alpha (tef1) and RNA polymerase second largest subunit (rpb2) gene sequences and conducted pathogenicity tests to fulfill Koch’s postulates. Our findings have important implications for the control of leaf spot disease in cannabis cultivation.