Large quantities of volatile radionuclides were released into the atmosphere after the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in March 2011. An inventory of these radionuclides in seafloor sediments is important for evaluating their environmental behavior and effects on aquatic organisms. We developed a dedicated radiation survey device that can be installed on a multipurpose unmanned surface vehicle called the Intelligent Boat for Oceanological Survey from Hama-dori (i-BoSH). Both the i-BoSH and dedicated radiation survey device were developed by a local consortium to survey places of heavy damage from the accident. The radiation survey device can measure radioactivity in-situ and collect eight sediment samples. The device also contains sensors to measure the relative distance between the i-BoSH and the seafloor as well as other fundamental seawater information such as temperature. A comprehensive test that included in-situ radiation measurements in sediment and collection of sediment core samples was conducted offshore from a radiocesium-contaminated fishing port in Fukushima. Consequently, the effectiveness of this device was confirmed via a successful operation at 12 sampling points with two cruises from the fishing port. Good correlations were observed between the laboratory and field measure-ment results of the sampled sediment cores and the in-situ radiation survey, re-spectively. Successful application of this device is expected to go beyond environmental surveys around the FDNPS to more general radiation monitoring around nuclear facilities and longer post-accident periods.