Lasianthus kailarsenii Poopath, Vajrodaya & Napiroon, L. tennissarimensis Napiroon, Balslev & Chayamarit and L. krabiensis Napiroon, Balslev & Chamchumroon are described as new species and illustrated. Lasianthus kailarsenii is morphologically close to L. lancilobus, but differs from it by its glabrous (not hirsute) branches and leaves, campanulate and glabrous (not obconical and hirsute) calyx, exserted (not included within the corolla tube) stigma, and 8 (not 4) pyrenes. Lasianthus tennissarimensis resembles L. robinsonii but differs from it by having lanceolate leaves that are 7.5–10 cm long (not oblong or oblong-elliptic, 12–16 cm long), its 5–6 (not 8–9) pairs of secondary veins, the calyx limbs that are narrowly lanceolate and 7–8 mm long (not linear and 2.5–4 mm long). Lasianthus krabiensis also resembles L. lancilobus but differs from it by having the abaxial leaf surface sparsely hirsute (glabrous), peduncle 1.5 mm (not 2.5–3 mm) long, and bracts shorter than calyx tube, 5–6 mm long (not longer than calyx tube, 1.5 cm long). Lasianthus oblongus which was previously known only from Peninsular Malaysia, has now been recorded in Yala (southern Thailand). A key that compares them to the related Thai Lasianthus species is also provided.