Studies have examined the impact of intergenerational mobility on individuals' socioeconomic, psychological, or health outcomes. However, little is known about its impact on feeling of loneliness, despite its increasing importance in recent years. Using survey data collected in 2022 Japan, we analyze the relationship between the experience of intergenerational educational mobility and current levels of loneliness for young and middle-aged individuals. Using diagonal reference models, which allow us to isolate the effect of mobility from the influence of origin and destination, we find that downward intergenerational educational mobility is significantly associated with greater loneliness, whereas upward mobility is not significantly associated. Moreover, the effect of downward educational mobility is stronger for men than for women. The results suggest that intergenerational mobility also has an impact on loneliness. We discuss the theoretical implications of these findings and conclude that the loneliness should be studied not only in terms of the individuals’ current situation but also in terms of their experiences and the social contexts.