We developed a new cultivar of Tartary buckwheat (Fagopyrum tataricum Gaertn.), 'Darumadattan'. This is the first semidwarf Tartary buckwheat cultivar to be developed by mutation breeding using gamma-ray irradiation. In 1999, 100 dry seeds of the leading Japanese cultivar, 'Hokkai T8' (known at that time as 'Hokkei 1'), were gamma-ray-irradiated with a total dose of 500 Gy (25 Gy/h × 20 h) at the Institute of Radiation Breeding (IRB), National Institute of Agrobiological Sciences, Hitachiomiya, Ibaraki, Japan. The seeds were sown in August 1999 in a field at IRB, and M2 seeds were collected from the eight individual plants that survived. In August 2000, 240 M2 seeds were sown in a field, and one semidwarf plant was found. The line named 'IRBFT-20' developed from the selected plant was investigated for its semidwarf characteristic and genetic stability in 2001-2005. 'IRBFT-20' was submitted for registration in 2011 and registered as the cultivar 'Darumadattan' in 2013. This name was chosen because the plants resemble "Daruma dolls" and "dattan" means "Tartary" in Japanese. 'Darumadattan' is a highly lodging-resistant and high-yielding cultivar and is expected to be used as both a commercial cultivar and a crossing parent. Copyright © 2020 by JAPANESE SOCIETY OF BREEDING.