Generalised oedema is characterised by tissue swelling due to excess water accumulation. It is a frequent complication encountered in infants admitted to paediatric and neonatal intensive care. In clinical practice, oedema assessment depends on a combination of weight monitoring, fluid balance calculations and direct visual inspection of the skin. However, all of these lack accuracy and sensitivity, especially in critically ill infants. We report the use of near infrared (NIR) spectroscopy using a portable spectrometer and an optical fibre with a custom attachment allowing continuous recordings for 30 min. Data were collected from 20 neonates and infants admitted to the intensive care unit, with or without oedema, across three consecutive days. Statistically significant differences were found in the 1450 nm features, amplitude (p = 0.009), ratio (p < 0.001) and band area (p < 0.001), and the 970 nm ratio followed the same direction (p = 0.005), all with higher values in the oedema group. A late fusion model, combining spectral data and clinical features, effectively differentiated between oedematous and non-oedematous infants, achieving AUC 0.909 ± 0.016, p = 0.004. This proof-of-concept study suggests that near-infrared spectroscopy can detect oedema in neonates and infants, as assessed against clinical evaluation, laying the groundwork for its potential application as a non-invasive clinical detection and monitoring tool.