Despite limited pharmacokinetic (PK) data, dexmedetomidine is increasingly being used off-label for sedation in infants. We aimed to characterize the developmental PK changes of dexmedetomidine during infancy. In this open-label, single-center PK study of dexmedetomidine in infants receiving dexmedetomidine per clinical care, ≤10 blood samples per infant were collected. A set of structural PK models and residual error models were explored using nonlinear mixed-effects modeling in NONMEM. Covariates including postmenstrual age (PMA), serum creatinine, and recent history of cardiac surgery requiring cardiopulmonary bypass were investigated for their influence on PK parameters. Univariable generalized estimating equation models were used to evaluate the association of hypotension with dexmedetomidine concentrations. A total of 89 PK samples were collected from 20 infants with a median PMA of 44 weeks (range, 33-61). The median maximum dexmedetomidine infusion dose during the study period was 1.8 μg/(kg·h) (0.5-2.5), and 16/20 infants had a maximum dose >1 μg/(kg·h). A 1-compartment model best described the data. Younger PMA was a significant predictor of lower clearance. Infants with a history of cardiac surgery had ∼40% lower clearance compared to those without a history of cardiac surgery. For infants with PMA of 33 to 61 weeks and body weight of 2 to 6 kg, the estimated clearance and volume of distribution were 0.87 to 2.65 L/(kg·h) and 1.5 L/kg, respectively. No significant associations were found between dexmedetomidine concentrations and hypotension. Infants with younger PMA and recent cardiac surgery may require relatively lower doses of dexmedetomidine to achieve exposure similar to older patients and those without cardiac surgery.