Background: Despite the surge in use of extracorporeal membrane oxygenation (ECMO) in the adult intensive care unit, little guidance is available on the appropriate dosing of antimicrobials in this setting. Ceftriaxone is an antimicrobial with a high affinity to plasma protein, a property identified in the literature as susceptible to sequestration into extracorporeal circuits and hypothesised to require dosage adjustments in this setting. Objective: The aim of this study was to describe the pharmacokinetics of ceftriaxone and identify the best dosing regimen for critically ill adult patients receiving ECMO. Methods: Serial blood samples were taken from patients receiving both ECMO and ceftriaxone. Total and unbound drug concentrations were measured in plasma by chromatographic assay and analysed using a population pharmacokinetic approach with Pmetrics®. Dosing simulations were performed to identify the optimal dosing strategy: 60 and 100% of time with free (unbound) drug concentration exceeding the minimum inhibitory concentration (fT>MIC). Results: In total, 14 patients were enrolled, of which three were receiving renal replacement therapy (RRT). Total and unbound ceftriaxone was best described in a two-compartment model with total body weight, serum albumin concentrations, creatinine clearance (CrCL), and the presence of RRT included as significant predictors of pharmacokinetics. Patients not on RRT generated a mean renal clearance of 0.90 L/h, non-renal clearance of 0.33 L/h, and central volume of distribution of 7.94 L. Patients on RRT exhibited a mean total clearance of 1.18 L/h. ECMO variables were not significant predictors of ceftriaxone pharmacokinetics. Steady-state dosing simulations found that dosages of 1 g every 12 h and 2 g every 24 h achieved >90% probabilities of target attainment in patients with CrCL of 0 mL/min with RRT and 30 and 100 mL/min and various serum albumin concentrations (17 and 26 g/L). Conclusions: Dosing recommendations for critically ill adult patients not on ECMO appear to be sufficient for patients on ECMO. Patients exhibiting augmented renal clearance (> 130 mL/min) or treatment of less susceptible pathogens may require higher doses, which requires further investigation.