Background: Dietary supplements are widely used by cancer survivors. However, health effects among older cancer survivors are unclear. Methods: We used the Iowa Women's Health Study, a prospective cohort study with 2,118 postmenopausal women with a confirmed cancer diagnosis (1986-2002), to evaluate the association between postdiagnosis dietary supplement use assessed in 2004 and subsequent all-cause mortality. Risk of death was evaluated using multivariable-adjusted Cox proportional hazards regression. We performed stratified analyses by diet quality score, dietary micronutrient intake, and perceived general health. Results: Through 2010, 608 deaths were identified. Approximately 85% of the cancer survivors used dietary supplements. Overall supplement use and multivitamin use were not associated with mortality. Iron supplement use was associated with 39% higher risk of death [95% confidence interval (CI), 1.09-1.77]. This association was stronger among survivors with deteriorating general health. Folic acid supplement use was associated with higher risk of death, only among survivors reporting low-quality diets (HR, 2.33; 95% CI, 1.33- 4.08; Pinteraction = 0.006). Multivitamin use and using a greater number of supplements was associated with a trend towards higher mortality only among those with poor diet quality. Using vitamin E supplements in combination with multivitamin was associated with lower risk of death only among survivors with higher dietary vitamin E intake (HR, 0.61; 95% CI, 0.39-0.94; Pinteraction = 0.02). Conclusions: Postdiagnosis supplement use was associated with higher mortality among older female cancer survivors with poor general health and/or poor dietary intake. Impact: The association between postdiagnosis dietary supplement use and mortality may differ by diet quality and health status among older female cancer survivors.