ROV dive surveys were carried out inside and outside the Sumisu Caldera, located in the Izu-Bonin Arc. The caldera is hydrothermally active and nourishes a unique chemosynthetic ecosystem, which includes Bathymodiolus mussel beds and vestimentiferan tubeworms. Sixty-one gelatinous zooplankton morphotaxa were observed (21 ctenophores, 16 siphonophores, 10 hydromedusae, 4 scyphozoans and 10 thaliaceans), and notes on their taxonomy and fine-scale distributional data are presented. The vertical distribution patterns of gelatinous zooplankton clearly differed inside and outside the caldera: three gelatinous zooplankton morphotaxa, the ctenophores Lobata sp. "Boli" and undescribed Lobata "No auricles", and the hydromedusa Earleria bruuni, were highly abundant inside, but not outside, of the caldera. Thaliaceans and Solmissus incisa s.l. (Narcomedusae) were distributed over a wider vertical range inside the caldera than outside. The utility of ROV video records for investigating midwater gelatinous zooplankton taxonomy and ecology is discussed, and the efficacy of ROV investigations for this type of research is shown.