Household food insecurity contributes to poor nutritional health, with negative consequences on growth and development during childhood. Although early childhood nutrition needs have received much attention, another important nutritional phase is adolescence. In a sample of 670 adolescents from Kilosa District, Tanzania, this study used 3 approaches to better understand the relationship between food insecurity and undernutrition. First, this study examined the associations between 3 commonly used measures of household food security and undernutrition among 670 adolescents from Kilosa District, Tanzania. The measures of household food security, energy adequacy per adult equivalent, dietary diversity score, and coping strategies index, were strongly correlated with each other and household assets (P < 0.05). Second, this study measured the nutritional status of adolescents in this district, finding a high prevalence of undernutrition (21% with BMI-for-age <5th percentile of the National Center for Health Statistics/WHO reference). Third, this study measured the association between the log odds of undernutrition (as the dependent variable) and each of the 3 measures of household food security. In separate models, household energy adequacy per adult equivalent and household dietary diversity score were inversely associated with undernutrition after adjusting for gender, age, puberty, and the interaction between age and puberty. By contrast, a greater use of coping strategies was not associated with undernutrition. Strategies focused on increasing household energy intake and improving dietary diversity among the most vulnerable households could improve the nutritional health of adolescents.