The small but distinctive cubomedusa Carybdea sivickisi has been reported from a range of tropical, subtropical, and mild temperate localities in the Pacific. In Japan, it has only definitively been documented in the subtropical region of Okinawa. However, in 1970 Uchida noted that three specimens from Seto, Wakayama, which he had referred to as Tamoya alata in 1929, were really C. sivickisi. The presence of this species as far north as Wakayama was questionable because of the confusion about the identity of the specimens and the fact that C. sivickisi had never been observed subsequently in this region. In August 2006, we discovered a population of this tiny species nearby the Seto Marine Biological Laboratory in Shirahama, Wakayama. In this paper we comment and clarify the geographical range, developmental morphology, and sexual behaviour of this species.