This paper explores the eugenic thought of Yamanouchi Shigeo (1876–1973), who was trained in plant cytology under the tutelage of botanist and eugenicist John Coulter (1851–1928) in the USA, and later became one of the early and important popularizers of eugenic ideas in Japan. His career demonstrates a direct link between Japanese and US eugenics. Despite his academic training and research at various internationally renowned institutions, numerous publications, and longevity, his life has received little scholarly attention. By the early twentieth (...) century, most biologists in Japan, as in the USA, began accepting Mendelian evolutionary theory and rejecting the Lamarckian notion of inheritance of acquired characteristics. However, Yamanouchi Shigeo's eugenic view represents a paradox: he was a Mendelian cytologist sympathetic to Lamarckism. Was his ‘nurture’‐oriented eugenic view unscientific? Is that why he was largely ignored in the history of botany in Japan? This study attempts to answer these questions and to analyse the origins and distinct features of Yamanouchi's eugenic ideas by situating Yamanouchi's eugenic thought historically and culturally. After examining his scientific papers, popular writings, and documents of various organizations to which he belonged, I argue that Yamanouchi's ‘softer’ (or less biologically deterministic) perspective may have reflected the Japanese desire to catch up with the dominant ‘race’ by using eugenics without accepting permanent inferior status. (shrink)