A new species of the green algal genus Rhipidosiphon, Rhipidosiphon lewmanomonnae sp. nov., is described after material from the Mu Ko Tha Lae Tai area (SE Thailand) and Okinawa (Japan) on the basis of morphology and molecular analyses. The new species grows epilithically, just under the low water mark and is occasionally exposed at spring low tide. It differs mainly from the other known species, the Indo-Pacific Rhipidosiphon javensis and the western Atlantic Rhipidosiphon floridensis, by the presence of a rhizoidal pseudocortex surrounding the monosiphonous stipe and the presence of intercalary siphon constrictions. A similar pseudocortex has been described in juvenile stages of some Udotea species, but in this genus, the stipe is polysiphonous and possesses a cortex of lateral appendages in the adult phase.