Paleogene faunas from Southeast Asia are rare and mainly represented by those from Krabi (late Eocene, Thailand) and Pondaung (middle Eocene, Myanmar). The late Oligocene locality of Nong Ya Plong (Thailand) was discovered recently and has so far yielded mammalian remains representing the orders Carnivora, Dermoptera, Rodentia, and Perissodactyla. A new genus and species from this locality (Siamosorex debonisi n. gen., n. sp.) of the soricoid family Plesiosoricidae is described. The material consists of lower dentitions only. The species differs from other plesiosoricids in being larger to much larger (except compared to some species of Plesiosorex) and in having a shallow masseteric fossa; i2 enlarged, not procumbent but upright and caniniform, with a deep mesiolingual groove; elongated, double-rooted p3 and p4, similar in length but p3 with simple crown and p4 semimolariform with a reduced paraconid, a high, distinct metaconid, and a short talonid; lower molars decrease in size strongly from m1 to m3; elongated molars with the talonid markedly narrower and shorter than the trigonid, tall and open trigonid, especially in m1, entoconid and hypoconid not individualized, entocristid lower than the oblique cristid, and metaconid taller than paraconid in m1, subequal to the paraconid in m2. Siamosorex debonisi n. gen., n. sp. was approximately the same size as small species of Mustela (Carnivora) and probably preyed on small vertebrates and arthropods. There is some support for the presence of a venom delivery apparatus in S. debonisi n. gen., n. sp. The origin of Siamosorex n. gen. is unknown, although this genus may have some relationships with the genus Butselia from the early Oligocene of western Europe.