This study reports the development of a new pure white Flammulina velutipes variety, ‘Seoldan’. The breeding process involved a primary cross between a brown strain (KMCC02267) collected by the Rural Development Administration and a white cultivar ‘Baekwoon’ (KMCC05472). From this cross, a white line ‘Fv-23-553’ was selected and subsequently crossed with a superior pure white resource (KMCC05474), resulting in the novel cultivar Seoldan. Mycelial growth tests showed that both varieties grew best at 25°C; however, Seoldan exhibited faster mycelial growth than the control variety under four temperature conditions except at 20°C. In media tests, Seoldan also demonstrated superior growth on three media types, with the exception of YM medium. In sawdust bottle cultivation, The time from scratching to the first fruiting was 2 days shorter in Seoldan than in the control variety, contributing to an overall reduction in cultivation time. The most distinct morphological trait of Seoldan is its pure white cap color. The average yield was 218.8 ± 10.6 g per bottle, comparable to that of the control. Morphologically, Seoldan produced smaller pileus and thinner stipes than the control. Furthermore, somatic incompatibility tests confirmed that Seoldan is genetically distinct from the control variety. These results indicate that Seoldan is a promising pure white variety with stable productivity, improved cultivation efficiency, and clear genetic differentiation from existing varieties.