OBJECTIVE: To investigate the relationship between serum ferritin levels and OSA in overweight/obese individuals and assess the association between ferritin levels and all-cause mortality in overweight/obese female OSA patients. METHODS: Cross-sectional (n = 4,809) and prospective (n = 832) data from overweight/obese adults from the 2005-2008, and 2015-March 2020 NHANES cycles were analyzed. Participants were categorized into quartiles (Q1-Q4) based on their ln-transformed serum ferritin levels. Multivariable logistic regression and restricted cubic spline regression (RCS) investigate associations. Kaplan-Meier survival analyses and Cox proportional hazards regression examined the relationship between ferritin levels and all-cause mortality in OSA patients. RESULTS: After adjusting for potential confounding factors, we found that ln-transformed ferritin levels were associated with an increased risk of OSA (Q4 vs Q2: OR = 1.07, 95% CI: 1.01-1.13, P = 0.020, P for trend = 0.010). A non-linear U-shaped association was observed between ferritin levels and OSA risk (P-non-linear = 0.029), with an inflection point at ln-transformed ferritin of 4.58 (corresponding to a serum ferritin concentration of 97.51 ng/mL). In female OSA patients, elevated ferritin levels were associated with increased all-cause mortality risk (Q4 vs Q2: HR: 5.46, 95% CI: 1.18-25.16, P = 0.029, P for trend = 0.032). CONCLUSION: Ferritin levels in overweight/obese individuals show a U-shaped relationship with OSA risk, and elevated levels correlate with increased all-cause mortality in female overweight/obese OSA patients. In the future, further research is needed to explore the potential associations between ferritin, inflammation, obesity, and OSA.