In March 2019, fruit rot was observed on approximately 25% of melons (Cucumis melo) collected in Wang Thong, Phitsanulok Province, Thailand. The decayed area started from the base of fruit, appearing as black spots surrounded by a dark brown margin. The internal decayed area appeared rotten and was surrounded by water-soaked tissues. The causal pathogen was isolated and identified as Lasiodiplodia theobromae based on morphological characteristics, molecular data (sequence analysis of the internal transcribed spacer (ITS) region of rDNA, β-tubulin (TUB) gene, and translation elongation factor-alpha (TEF-1) gene) and pathogenicity test. This is thought to be the first report of L. theobromae causing postharvest fruit rot on melons in Thailand.