<p>Chronic kidney disease of unknown etiology (CKDu) affects young adults in rural Mesoamerica, but environmental drivers remain unclear. Per- and polyfluoroalkyl substances (PFAS) are persistent nephrotoxic contaminants, yet evidence from CKDu-endemic regions is limited. This study assessed serum PFAS concentrations and their association with kidney function in young adults in northwestern Nicaragua. A nested cross-sectional study was conducted within a community-based cohort study in northwestern Nicaragua. Serum samples from 85 young participants (18–30&#xa0;years) were selected in year four and stratified by glomerular filtration rate (eGFR). Targeted and non-targeted PFAS were measured using liquid chromatography–quadrupole time-of-flight mass spectrometry. Sex-stratified linear regression models assessed associations with eGFR, adjusting for age, body mass index, and smoking. Six PFAS were detected in &gt; 50% of participants. PFOS and PFOA were most prevalent, with higher concentrations in men. Elevated concentrations were observed among workers in banana plantations, commerce, and construction. PFOS was positively associated with eGFR (β = 1.12&#xa0;mL/min/1.73m² per µg/L; 95%CI: 0.14, 2.10), with no other significant associations. This study provides the first PFAS biomonitoring data from a CKDu-endemic region in Mesoamerica. While most PFAS showed no association with kidney function, the sex-specific PFOS–eGFR association in men warrants further longitudinal investigation. Chronic kidney disease of unknown etiology (CKDu), Per- and polyfluoroalkyl substances (PFAS), Kidney function, Longitudinal study, Nicaragua</p>