During a revision of Radulaceae in Southeast Asia, an undescribed species of the genus Radula was discovered from Thailand and Brunei Darussalam. Morphological characters were examined from fresh and herbarium specimens using stereo and compound microscopes. Phylogenetic analyses were conducted based on six chloroplast markers (atpB–rbcL, psbT–psbH, psbA–trnH, rps4, trnG, and trnL–F) using Maximum Likelihood and Bayesian Inference approaches. Radula rheophila is described and illustrated as a new species from peninsular Thailand and Brunei Darussalam. The new species is characterized by female bracts usually in 2 or 2.5 pairs unequal pairs; longitudinally rectangular or ligulate leaf lobules when large, broadly transversely rhombic or rectangular when small, 1/3–2/5 the length of the lobe; ovate to broadly ovate leaf lobes with broadly obtuse to rounded apices and entire margins; finely botryoidal oil bodies (2–3 per cell); and stems with thickened exterior cortical walls and thin-walled interior cortical and medullary cells thickened at the corners by concave trigones. Phylogenetic analyses strongly support its placement within Radula subg. Odontoradula. The discovery of R. rheophila enhances understanding of morphological variation and phylogenetic relationships within Radula, highlighting the significance of integrative taxonomy for uncovering tropical bryophyte diversity.