This study employed inter-simple-sequence repeat (ISSR) to assess genetic variation among 189 individuals representing 10 populations (nine in Korea and one in Japan) of Millettia japonica, which has recently been lifted from the endangered species of Korea. The calculated Shannon`s information index value (I = 0.2689) of the species was appreciable and was higher than other endangered leguminous woody taxa. Gochang (I = 0.2968), Namhae (I = 0.2951), and Mt. Toham (I = 0.2823) populations showed relatively high genetic diversity, whereas the Kyushu (in Japan) population (I = 0.2487) exhibited the lowest. The results of an analysis of molecular variance indicated that 86.49% of the diversity was attributed to within populations, and 13.51% to differences among populations, suggesting that M. japonica populations do not have significant geographic differentiation and that the gene flow between populations exists to some extent (Nm = 1.8446). Continuous habitat monitoring should be conducted to conserve genetic diversity of M. japonica, particularly for those populations with relatively high genetic diversity. Selection of many individuals from the populations in Gochang, Namhae, and Mt. Toham is thought to be an appropriate strategy for ex situ conservation of M. japonica in Korea.