A new species of worm eel (family Ophichthidae: subfamily Myrophinae), Mixomyrophis longidorsalis sp. nov. is described based on a single specimen [77.0 mm in total length (TL)] collected from the Red Sea. The new species is similar to Mixomyrophis pusillipinna, its only congener, in preanal and total vertebral counts. However, the former species can be clearly distinguished from the latter by its shorter trunk (22.3 % TL vs. 28.4 %), shorter predorsal length (16.3 % TL vs. 23.1 %), more numerous counts of lateral-line pores before gill opening (12 vs. 9), and predorsal vertebrae (22 vs. 33). The holotype of M. longidorsalis is considered to be a juvenile, having grown from metamorphosis due to the following information: it is presumed that M. pusillipinna metamorphoses at 65–80 mm TL and the sizes at metamorphosis of congeneric myrophines are almost the same; the holotype of M. longidorsalis lacks larval pigmentation on its body and has numerous pale dots on the surface of the body. Although the lateral-line system of the holotype does not reach the level of mid-anus, it is unclear whether the holotype shows the adult condition. This new species extends the range of Mixomyrophis to include Atlantic and the Indian Ocean.