The prevalence of multigenerational families is on the rise in the United States, as is food insecurity. We estimate the effect of resident grandchildren on the risk of and transitions in food insecurity using repeated cross sections and longitudinally linked two-year panels of the Current Population Survey from 2001-2010. We find that rates of food insecurity in families with a grandchild present are at least twice as high in a typical year compared to families without a resident grandchild, and the extent of very low food security increased substantially faster among these households over the past decade. The rise in food insecurity during and after the Great Recession is due to both increased entry into food insecurity and decreased exit out of food insecurity. A similar trend accounts for the rise in multigenerational households during the recession—grandchildren were more likely to move in with their grandparents, and once there, were less likely to move out. There are also important differences in risk factors for food insecurity between multigenerational families and those with no grandchildren present. Our transition models show that whether grandchildren remain, or in periods of transition, multigenerational families are at heighted risk of entering food insecurity and remaining in this state. However, the entry of a grandchild may not always be a negative for the family’s food security, nor the exit of the child a positive. Entrance of a child seems to buffer the family from extreme forms of food insecurity while exit exposes the family to risk of deeper food insecurity.