Hydrozoan jellyfish of the genus Blackfordia currently consist of three described species: B. manhattensis, B. virginica and B. polytentaculata. These species are distinguished by combinations of the position and shape of the gonads, the number of marginal tentacles, and the presence or absence of black pigments on the bell rim. We discovered a species of the genus Blackfordia in Vietnam for the first time. Morphological and molecular analyses revealed that this Vietnamese Blackfordia had an intermediate number of tentacles compared with its congeners, and a similar number of tentacles had been reported in Blackfordia only in Brazil, Mexico and India. Mitochondrial DNA analysis indicated that Vietnamese Blackfordia formed a sub-clade with "B. virginica" from Brazil, and differed from B. virginica from other locations, having a K2P distance of >0.13. These results suggest that Blackfordia sp. found in Vietnam (and Brazil) may be a different species from B. virginica found elsewhere, though more information is needed to establish this new species.