Introduction: Acute kidney injury (AKI) is associated with the abrupt loss of kidney function resulting in increased morbidity and mortality. Oxidative stress plays an important role in the pathophysiology of AKI. Free thiols (R-SH, sulfhydryl groups) are crucial components of the extracellular antioxidant machinery and reliably reflect systemic oxidative stress. Lower levels of thiols represent higher levels of oxidative stress. In this study, we hypothesized that plasma free thiols are associated with AKI upon admission to the intensive care unit (ICU). Methods: In this study, 301 critically ill patients were included for analysis. Plasma samples were taken upon admission. Plasma levels of albumin-adjusted plasma free thiols were determined and correlated with AKI stage upon ICU admission. Results: Albumin-adjusted plasma free thiols were significantly lower in patients with AKI (n=43, median [interquartile range] 7.28 µmol/g [3.52,8.95]) compared to patients without AKI (8.50 µmol/g [5.82, 11.28]; p < 0.05) upon admission to the ICU. Higher age (B = -0.72, p < 0.001), higher levels of neutrophil gelatinase-associated lipocalin (B = -0.002, p < 0.05), creatinine (B = -0.01, p < 0.05) and lower serum albumin (B = 0.47, p < 0.001) were associated with lower free thiol levels. Further, albumin-adjusted free thiol levels were significantly reduced in patients with sepsis (8.30 [5.52-10.64] µmol/g) compared to patients without sepsis (6.95 [3.72-8.92] µmol/g; p < 0.05). Conclusion: Albumin-adjusted plasma free thiols were significantly reduced in patients with AKI and patients with sepsis compared with patients without AKI and sepsis. Together, these data suggest that free thiol levels are mainly reduced in sepsis-associated AKI.