Epidemiological evidence on peripheral iron and cognitive impairment in older adults is sparse and limited. Results on serum iron and cognitive impairment in older adults from the National Health and Nutrition Examination Survey have not been reported. Data on serum iron and cognitive impairment from individuals ≥ 60 years of age were obtained from the 2011-2014 NHANES (N = 3,131). Serum iron concentrations were determined with DcX800 method. Cognitive impairment was assessed with four cognitive tests: the Digit Symbol Substitution Test (DSST), the Animal Fluency (AF), the Consortium to Establish a Registry for Alzheimer's Disease Delayed Recall (CERAD-DR) and Word Learning (CERAD-WL) tests. Logistic regression and restricted cubic splines were adopted to explore the dose-response relationship between serum iron concentrations and cognitive impairment. Comparing the highest to lowest tertile of serum iron concentrations, the multivariate-adjusted odds ratios of scoring low on the DSST were 0.70 (0.49-1.00), 0.88 (0.65-1.20) for CERAD-WL, 0.65 (0.48-0.88) for CERAD-DR, and 0.78 (0.53-1.15) for AF. Stratified analyses by sex showed that the above-mentioned associations were mainly found in men; however, the interaction with sex was not significant. Dose-response analysis showed that relationships between serum iron and cognitive impairment evaluated by DSST and CERAD-DR were linear, respectively.