This study examines associations between grandparent involvement and change in parenting stress among nonmarried mothers with young children. It employs longitudinal data from the Fragile Families and Child Wellbeing Study. Grandparent involvement is assessed by a mother's reports of intergenerational coresidence and contact between her parents and her 1-year-old child. Self-reported parenting stress is assessed at the same time and, again, 2 years later. Findings from lagged-dependent regression models indicate that intergenerational coresidence is associated with decreasing parenting stress among Latina mothers but with increasing parenting stress among black mothers. Among mothers in nonintergenerational households, intergenerational contact is associated with decreasing parenting stress among black mothers but with increasing parenting stress among Latina mothers. No associations are found among white mothers. Findings suggest that sociocultural contexts pattern ways in which grandparent involvement protects or increases the vulnerability of nonmarried mothers with young children.