Purpose: Using urinary indices as a quick bedside test to assist management of oliguria and acute kidney injury (AKI) has long been sought. This study assessed whether urinary potassium excretion is related to simultaneously calculated creatinine clearance (CrCl) and can predict AKI in the critically ill. Materials and methods: In this prospective cohort study, the correlation between 2-h urinary potassium excretion and simultaneously calculated CrCl of 61 critically ill patients was assessed by Pearson's correlation coefficient, and their ability to predict AKI (≥stage 1 KDIGO) in the subsequent 7 days was assessed by area under the receiver-operating-characteristic (AUROC) curve. Results: Urinary potassium excretion (median 6.2 mmol, range 0.8–24.3) correlated linearly with CrCl (correlation coefficient: 0.58, 95% confidence interval [CI] 0.38–0.72; p = 0.001), and had a moderate ability to predict subsequent AKI (n = 19 [31%]; AUROC 0.747, 95%CI 0.620–0.850; p = 0.001), especially in patients without prior exposure to furosemide within 24-h (correlation coefficient 0.61, 95%CI 0.41–0.76; AUROC 0.789, 95%CI 0.654–0.890; p = 0.001, respectively). Conclusions: Urinary potassium excretion correlates with CrCl and predicts AKI in the critically ill without recent furosemide exposure. Given 2-h urinary potassium excretion can be measured easily, its potential as a marker of renal function deserves further study.