Two new species of free-living nematodes of the order Monhysterida have been described from bottom sediments of the Van Uc River, Vietnam. Amphimonhystrella major sp. n. resembles A. parva Gagarin, Nguyen Vu Thanh, 2009 by the structure of the spicular apparatus, but differs by a longer body, shorter tail, longer stoma, and larger spicules. Thalassomonhystera lepta sp. n. is morphologically close to T. elegans Gagarin, Nguyen Vu Thanh, 2013, but has a shorter body, smooth cuticle, tail of different shape, and larger spicules. A table of the main morphometric characters of males of the valid species of the genus Amphimonhystrella and a dichotomous key for their identification and the table of the main morphometric characters in males of seven valid species of the genus Thalassomonhystera found in water bodies of Vietnam and a dichotomous key for their identification are presented.