During preparation of a revision of the genus Eranthemum (Acanthaceae), it became evident that nine specimens from eastern and south-eastern floristic regions of Thailand and southern Laos, represent a new species, that is herein described, illustrated and given the name, E. decumbens. The new species has affinities to E. strictum and E. roseum. A key to the species occurring in Thailand is provided, together with ecological data, distribution map and conservation status.

The study is based on careful examination of morphological characters on specimens from the Thai herbaria BK, BKF, CMUB and QBG. In addition, images of herbarium specimens were examined from P. The taxonomic literature of the genus Eranthemum of various countries in Asia including India (Roxburgh 1820, Miquel 1858, Clarke 1885), Bhutan (Wood 1994, 2001), China (Hu  et  al. 2011), Myanmar (Kress  et  al. 2003), Thailand (Imlay 1938, 1939, Hansen 1985, Charoenchai 2014, The Forest Herbarium, Royal Forest Dept 2014), Laos (Newman et al. 2007) and Indo-China (Benoist 1935) was consulted. A detailed description and line drawing were prepared based on herbarium specimen observations. The distribution, habitat and phenology were recorded from the specimen labels. The IUCN red list assessments were performed based on the extent of occurrence (EOO) and the area of occupancy (AOO) in the program R ver. 3.5.1 (www.r-project.org).