A new subspecies, Ardisia crenata Sims subsp. mukdahanensis Chatan & Promprom (Primulaceae), is described from Mukdahan Province, Thailand. This discovery was made during field studies conducted from 2021 to 2023, during which the authors collected and analysed specimens, comparing them with relevant taxonomic literature and herbarium collections. A. crenata subsp. mukdahanensis is distinct from its closest relative, A. crenata subsp. crassinervosa by a combination of morphological traits, such as moderately dense minute hairs on young shoots, abaxial side of young lamina and calyx, peduncles and pedicels; larger flowers (7.0–7.5 mm) that are pure white or pinkish; larger fruits (7–8 mm) and absence of glandular punctation in organs such as lamina, calyx, corolla, anther and fruit. This new subspecies grows in slightly dense dry evergreen forests or open areas near streams and is preliminarily assigned to the Data Deficient (DD) category according to IUCN guidelines.