BACKGROUND: Cancer-related deaths and environmental issues pose significant global challenges. The Planetary Health Diet (PHD) is a healthy dietary pattern that simultaneously promotes human health and ecology. This study aims to investigate the association between the Planetary Health Diet Index (PHDI) and mortality among cancer survivors, as well as the mediating role of inflammation between PHDI and all-cause mortality. METHODS: This study analyzed data from 3,442 cancer survivors enrolled in the United States National Health and Nutrition Examination Survey between 1999 and 2018. To investigate the association between PHDI and mortality, we applied weighted multivariate Cox proportional hazards regression, restricted cubic spline analysis, subgroup analysis, and sensitivity analysis. The mediating effects of the Systemic Immune-Inflammation Index (SII) and Neutrophil-to-Lymphocyte Ratio (NLR) were assessed using the bootstrap method with 1000 simulations. RESULTS: In the fully adjusted model, each 10-point PHDI increase correlated with a 9% decrease in all-cause mortality (HR, 0.91
95% CI, 0.86-0.95), a 10% decrease in cancer mortality (HR, 0.90
95% CI, 0.83-0.99), and a 10% decrease in non-cancer mortality (HR, 0.90
95% CI, 0.85-0.96). The PHDI was significantly inversely correlated with SII and NLR, which were positively related to all-cause mortality. The mediation proportions of SII and NLR between the PHDI and all-cause mortality were 6.52% and 8.52%, respectively. CONCLUSIONS: Adherence to the PHD is associated with reduced all-cause, cancer, and non-cancer mortality among cancer survivors. Additionally, SII and NLR may mediate the relationship between PHDI and all-cause mortality.