Hypoxic-ischemic brain injury (HIBI) is a leading cause of death and lifelong disability in near-term and term infants, most often caused by perinatal arterial ischemic stroke (PAIS, incidence approximately 1:5000 live births) or perinatal asphyxia (PA, incidence 1-2:1000 live births). No treatment currently exists to repair or limit HIBI, and many affected infants go on to develop cerebral palsy, cognitive impairment, epilepsy, or visual or hearing deficits. Intranasally administered bone marrow-derived mesenchymal stromal cells (IN-MSC) are a candidate regenerative therapy for HIBI. Preclinical studies show that IN-MSC migrate to injured brain regions, dampen neuroinflammation, and stimulate endogenous neural repair, improving both structural and functional outcomes in animal models of HIBI. A first-in-human phase I trial (PASSIoN) in 10 neonates with PAIS showed that intranasal MSC administration was feasible and well tolerated, with no serious adverse events and promising early efficacy signals at 2-year follow-up. iSTOP-CP is a phase II, single-center (UMC Utrecht, the Netherlands), double-blind, randomized, placebo-controlled trial evaluating whether IN-MSC therapy can prevent or reduce motor and other disabilities in infants with MRI-confirmed HIBI due to PAIS or PA. Eligible infants are born at 35.0 weeks of gestation or later, have HIBI confirmed on brain MRI or MRS in predefined brain regions predictive of poor outcome, and are diagnosed with PAIS or PA, with or without prior therapeutic hypothermia. A total of 162 infants will be randomized 1:1 to IN-MSC or matching placebo, stratified by the cause of HIBI and, within the PAIS group, by stroke territory. Infants with suspected chromosomal, metabolic, or congenital central nervous system anomalies, intracranial hemorrhage as the main injury, or contraindications to intranasal administration are excluded, as are infants for whom the clinical team has decided to withdraw intensive care. Participants receive a single intranasal dose of bone marrow-derived MSC or a visually indistinguishable placebo within 7 days after birth. Participants are followed for 24 months, with neurodevelopmental follow-up visits at 3-4, 6, 9-12, and 24 months of age and a brain MRI at 52 weeks postmenstrual age. The primary endpoint is the motor composite score of the Dutch version of the Bayley Scales of Infant and Toddler Development, 4th edition (Bayley-IV-NL), assessed at 24 months corrected age. Key secondary endpoints include cognitive development, the incidence of sensorineural disability, neuroregenerative imaging biomarkers, safety, health-related quality of life for infants and their parents, and the cost-effectiveness of IN-MSC treatment, evaluated through a health technology assessment.