Endolichenic fungi associated with lichen thalli in Thailand are poorly known in terms of species diversity. During a study conducted in Chiang Mai Province, Thailand, in 2023, eight endolichenic fungal strains were isolated from healthy thalli of the foliose lichen, Parmotrema sp. These eight strains were identified as members of the family Herpotrichiellaceae using a combination of three nuclear ribosomal regions (ITS, LSU, and SSU), tub2 sequence data, and morphological characteristics. Two strains of Atrokylindriopsis racemosospora and three strains of Veronaea endolichena were identified as new species within the Herpotrichiellaceae, while three other strains were identified as the previously known species Phialophora chinensis. This study provides the first report of P. chinensis as an endolichenic fungal taxon and its first discovery in Thailand. Descriptions, illustrations, and phylogenetic placements of these eight strains are provided. Additionally, a discussion and update on the ecology and genera within the Herpotrichiellaceae are included. The findings of this study offer valuable information that enriches the diversity of endolichenic species associated with lichens in Thailand and contributes to enhancing our understanding of the ecology and taxonomy of the Herpotrichiellaceae.