The 15th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation and Safety (NUTHOS-15) will take place in Kashiwa-no-ha, Japan from September 27-30, 2026. The meeting will be hosted by the Japan Society of Atomic Energy (AESJ) and co-hosted by the American Nuclear Society (ANS), the European Nuclear Society (ENS), the Korean Nuclear Society (KNS) and Canadian Nuclear Society (CNS). Kashiwa-no-ha is an innovative science city located approximately 30 km north of Tokyo and 30 km south of Tsukuba, offering convenient access from major airports as well as to numerous attractions and natural areas. NUTHOS has been one of the major conferences in the nuclear thermal-hydraulics, operation and safety. The first NUTHOS was held in Taipei in 1984 followed by Tokyo (1986), Seoul (1988), Taipei (1994), Beijing (1997), Nara (2004), Seoul (2008), Shanghai (2010), Kaohsiung (2012), Okinawa (2014), Gyeongju (2016), Qingdao (2018), Hsinchu (2022) and Vancouver (2024). NUTHOS-15 offers various technical tracks including Thermal-Hydraulics and Computational Methods, Experimental Thermal-Hydraulics, Safety and Severe Accidents, Thermal-Hydraulics and Safety of Advanced Reactors, Plant Operation and Maintenance, Diagnostics and Monitoring, SMRs and Micro Reactors, and AI applications. It will be a valuable opportunity to exchange ideas, connect with colleagues from around the world, and help shape the future of nuclear science andengineering.