The jellyfish Nemopilema nomurai has occurred in large numbers in Japan and Korea and has had on a negative effect on coastal fisheries in this region. Data on the abundance and distribution of jellyfish are needed to forecast when and where they will occur in coastal areas. Acoustic techniques are commonly used to study the distribution and abundance of fish and zooplankton. However before such surveys can be conducted, the acoustic characteristics of the target species must be known. In this study, the density of and speed-of-sound in jellyfish were measured to clarify their acoustic characteristics using a theoretical scattering model. The acoustic characteristics were estimated with the distorted-wave Born approximation (DWBA) model using these material properties and the shape of free-swimming jellyfish.