Solenaia khwaenoiensis n. sp. (Bivalvia: Unionidae) is described from The Khwae Noi River, Phitsanulok, Thailand. Solenaia khwaenoiensis n. sp. has long razor-shell like shells similar to a former described Solenaia emarginatus (Lea, 1860) from “Siam”. The new species possesses an elongate linguiform shell but the narrow anterior and wide posterior confers a unique razor shell shape. The umbo is located towards the anterior near the adductor muscle scar. Solenaia khwaenoiensis exhibits distinct intrageneric differentiation in the diploid chromosome number (2n=37) but is uniform in haploid number (n=19), with 3 metacentric 15 submetacentric and 1 subtelocentric chromosomes.