Single mothers’ relatively high levels of poverty are well documented, but the role that intergenerational coresidence may play in mitigating this disadvantage is not well understood. In this paper, we use multiple rounds of a large national survey between 1986 and 2007 (n = 67,252) to evaluate the extent to which income sharing via intergenerational coresidence limits poverty among single mothers in Japan. Results indicate that conventional poverty rates based on single-mother households overstate the prevalence of poverty among single mothers by 12–20 percent as a result of excluding those who are coresiding with parents. We find no evidence that the prevalence of intergenerational coresidence has changed in a way that would offset the poverty-increasing effect of growth in single parenthood. Finally, we show not only that shared income is the most important factor in limiting poverty among single mothers living with parents, but also that many single mothers are coresiding with parents who are themselves economically disadvantaged. We discuss the implications of our findings for understanding relationships between rapid family change and poverty in countries like Japan, where public income support for single mothers is limited and where family support via intergenerational coresidence is both common and normative.