Radula deflexilobula Promma, L.N.Zhang et R.L.Zhu is described and illustrated as a new species from Thailand. This species is distinctive in 1) the presence of dimorphic leaf lobules that are subrhombic and strongly reflexed on pendulous shoots, subquadrate and not reflexed on creeping shoots; 2) leaf lobules covering the stem ca. 1/3-1/2 of the stemwidth, not auriculate at base, and 3) leaf cells thin walled with indistinct trigones and smooth cell surface. Our phylogenetic analyses based on three chloroplast regions (trnG, trnL-F, atpB-rbcL) confirm that this new species belongs to subg. Radula and is sister to R. decurrens Mitt. known from Fiji.