A new species of smooth clam shrimp (Branchiopoda: Laevicaudata) from Mongolia and China is described here based on both morphological and genetic differences. The new species, Lynceus grossipedia n. sp., has unique features, including asymmetrically modified male thoracopods (left side thoracopods III-VI), male claspers 'movable finger' (=endopod) with delicate setation, and broad, bicarinate male and female rostrum. Lynceus grossipedia n. sp. is compared with the genera Paralimnetis and Lynceiopsis and a recently described Lynceus from China, also showing modified male thoracopods. Lynceus mandsuricus is declared nomen inquirendum. DNA barcoding is commonly used for speeding up species identification and systematics, but it has not been applied on smooth clam shrimp taxonomy yet. In order to explore the usefulness of DNA barcoding for smooth clam shrimps, we generated new cytochrome c oxidase (COX1) data for 10 Lynceus species. Previous Laevicaudata sequences in Genbank were scarce (~50) and biased, with 62% (n=31) being assigned to a single taxon (i.e., Lynceus macleayanus) and 28% (n=14) not assigned to species. Based on the addition of new barcoding data and the comparison with Genbank data for other clam shrimps, we could conclude that there is a similar distance threshold (=barcoding gap) between species of all three suborders (Spinicaudata, Cyclestherida, and Laevicaudata).