Hyperosmolar-hypernatremic dehydration (HHND) is a life-threatening yet preventable neonatal condition, often due to inadequate breastfeeding. The recent North Indian heat wave heightened dehydration risks, necessitating an evaluation of extreme temperatures' impact on neonatal hydration. This retrospective study analysed neonates admitted to a tertiary care level 3 neonatal intensive care unit (NICU) at AIIMS Jodhpur between April and June 2024. Case records were reviewed, and details on maternal age, feeding practices, presenting complaints, biochemical profile, and outcome were studied. The 2024 (April-May) heat wave led to a threefold increase in NICU admissions for HND compared to the previous 2 years, with cases rising from 2 to 3 per year to 10. Primigravida mothers accounted for 70% of the cases. The mean age of presentation was 6.7 days. Affected neonates experienced weight loss ranging from 11% to 33%, with serum sodium levels between 149 and 185 mEq/l and plasma osmolarity reaching 370-450 mOsm/l. Six neonates required peritoneal dialysis (PD) due to encephalopathy/anuria. One developed aortic thrombosis with lower limb gangrene, necessitating thrombolytic therapy. MRI abnormalities were observed in one case. Despite intensive management, one neonate succumbed to sepsis. Extreme environmental heat significantly heightens the risk of hyperosmolar-hypernatremic dehydration (HND) in neonates. Proactive neonatal monitoring, early breastfeeding support, and parental education are critical to preventing dehydration and its complications, especially in tropical and resource-limited settings, where extreme heat, early discharge, and limited lactation support increase neonatal vulnerability. Judicious fluid management targeting plasma osmolarity and timely intervention with PD in severe cases can optimize survival and neurological outcomes, underscoring the need for heightened vigilance during heat waves.