Trianthema portulacastrum is a medicinal herb and a leaf vegetable. In February 2023, leaf spots were observed in a horse purslane in Chiang Mai. The symptoms were initially observed as small brown spots on leaves starting from the margins which gradually spread towards the center and become irregularly shaped lesions. Pure culture was obtained from the diseased leaf using single-spore isolation. Pathogenicity test was carried out through detached leaf assay on wounded and unwounded conditions using spore suspension. The symptoms were observed three days after inoculation, small brown lesions were observed at the inoculation site of the wounded leaves. The pathogen was re-isolated from the symptomatic tissue and the pathogenicity was confirmed. The morphology revealed the pathogen represents Gibbago. The phylogenetic placement of the pathogen was obtained through concatenated internal transcribed spacer (ITS) region, large nuclear subunit ribosomal DNA (LSU), DNA dependent RNA polymerase II largest subunit; (RBP2), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Translation elongation factor 1-alpha (TEF-1α). Phylogeny revealed the pathogen as Gibbago trianthemae. In Thailand no phylogenetic study is available for the Gibbago trianthemae. Our study is the first phylogenetic study of Gibbago trianthemae from Thailand.