Summary Akebia quinata Decaisne is a monoecious woody vine. As the plants are self‐incompatible, separation of sex functions between flowers may function to avoid sexual interference rather than to avoid inbreeding depression. To investigate whether self‐pollen on stigmas affects outcrossed seed production in A. quinata, we carried out an artificial pollination experiment in which self‐pollination was conducted either simultaneously with, or 24 h before, cross‐pollination. Compared with cross‐pollination, fruit set was significantly reduced by prior self‐pollination. Simultaneous self‐ and cross‐pollination also reduced fruit production. Overall, self‐pollen on a stigma reduced seed production by 85% compared with cross‐pollination. Growth rate of self‐pollen tubes in carpels did not differ from that of cross‐pollen tubes, and self‐pollen tubes could reach ovules. This indicates that A. quinata has late‐acting self‐incompatibility, which may cause sexual interference if self‐pollination occurs. These results support the idea that monoecy in A. quinata functions to avoid sexual interference, which imposes a fitness cost, at least on female function.