The life cycle of a species of the genus Bougainvillia (Cnidaria: Hydrozoa), found in the southern Mediterranean Sea, Italy, is here described. Hydroid colonies produced immature medusae with two tentacles and two ocelli per bulb and four unbranched oral tentacles. The number of tentacles and ocelli, which remained constant during the entire life cycle, are here considered diagnostic characters to identify the present as a new species. Female medusae lived up to 47 days (with an average of 30 days) when reared at 17°C, while males, reared at the same temperature, were short-lived, concluding their life cycle in no more than 15 days. The medusa of this species resembles B. ramosa var. nana described by Hartlaub in 1911 on the basis of few specimens and no polyp stage. After the complete life cycle has been observed, and given its peculiar medusa stage, Hartlaub's subspecies (variant) must be promoted to species rank as Bougainvillia nana.