Introduction: The objectives of this retrospective cohort study were to examine the effect of vitamin K administration on hemorrhagic and thrombotic complications, blood product utilization, and outcomes in neonatal extracorporeal membrane oxygenation (ECMO). Methods: In the pilot study, complications, blood product use, and outcome data for neonates who received ( n = 21) or did not receive ( n = 18) a single dose of vitamin K (5 mg) immediately after initiation of ECMO for respiratory failure between 2006 and 2010 were compared. In the validation cohort, complications and outcomes were compared for 74 consecutive neonates supported with ECMO for respiratory failure who received ( n = 45) or did not receive ( n = 29) additional vitamin K once daily for prothrombin time (PT) ⩾14 seconds during ECMO from 2014 to 2019. Results: In the pilot study, vitamin K at ECMO initiation was associated with fewer thrombotic complications and similar hemorrhagic complications. The volume of fresh frozen plasma was higher in neonates who received vitamin K, but total blood product and other component volume did not differ between groups. ECMO run time, survival off ECMO, survival to discharge, and length of stay did not differ between cohorts. In the validation cohort, neonates who received additional vitamin K during ECMO had longer ECMO run time and length of stay, but no difference in mortality was observed. Further, thrombotic and hemorrhagic complications as well as blood product exposure were similar between cohorts. Conclusions: These data suggest that routine vitamin K administration may have limited or no benefit during neonatal ECMO.