Childhood adversity takes a toll on lifelong health. However, investigations of unpredictability as a form of adversity are lacking. Environmental unpredictability across multiple developmental periods and ecological levels was examined using a multiethnic, longitudinal birth cohort (1998-2000) oversampled for unmarried parents. Data were from the Future of Families and Child Wellbeing Study (N = 4898 youth at birth; 52% male; 48% Black, 27% Hispanic, 21% White) to examine unpredictability at ages 1, 3, 5, and 9 with later adolescent outcomes. An unpredictability index was associated with age 15 outcomes (N = 3595) including depressive symptoms (β = 0.11), anxiety symptoms (β = 0.08), delinquency (β = 0.13), impulsivity (β = 0.09), heavier weight categories (β = 0.09), and internalizing (β = 0.14), externalizing (β = 0.23), and attention problems (β = 0.16). Findings support unpredictability as a unique form of adversity.