Background: In sick neonates admitted to the NICU, improper fluid balance can lead to fluid overload. We report the impact of fluid balance in the first postnatal week on outcomes in critically ill near-term/term neonates. Methods: This analysis includes infants ≥36 weeks gestational age from the Assessment of Worldwide Acute Kidney injury Epidemiology in Neonates (AWAKEN) study (N = 645). Fluid balance: percent weight change from birthweight. Primary outcome: mechanical ventilation (MV) on postnatal day 7. Results: The median peak fluid balance was 1.0% (IQR: −0.5, 4.6) and occurred on postnatal day 3 (IQR: 1, 5). Nine percent required MV at postnatal day 7. Multivariable models showed the peak fluid balance (aOR 1.12, 95%CI 1.08–1.17), lowest fluid balance in 1st postnatal week (aOR 1.14, 95%CI 1.07–1.22), fluid balance on postnatal day 7 (aOR 1.12, 95%CI 1.07–1.17), and negative fluid balance at postnatal day 7 (aOR 0.3, 95%CI 0.16–0.67) were independently associated with MV on postnatal day 7. Conclusions: We describe the impact of fluid balance in critically ill near-term/term neonates over the first postnatal week. Higher peak fluid balance during the first postnatal week and higher fluid balance on postnatal day 7 were independently associated with MV at postnatal day 7.