Background/Objectives: The International Classification for Nursing Practice (ICNP®) is a standardized nursing terminology that enables the description of nursing care through diagnoses, interventions, and outcomes. An ICNP® Subset is a sub-group of ICNP® terms appropriate for settings of practice, facilitating the direct use of the ICNP® in nursing documentation. As far as we know, there are no Subsets concerning neonatology and Neonatal Intensive Care Unit (NICU) settings. The aim of this study is to develop a Subset of ICNP® NICU and neonatology setting, presenting terms that are validated and harmonized with SNOMED CT nomenclature. Methods: This is a two-phase study. In the first phase ICNP® terms were validated through a qualitative study using a four-round Delphi method and a focus group involving experts in NICU, neonatology settings, and education. The second phase focused on harmonizing the proposed ICNP® Subset with SNOMED CT. Results: A total of 479 ICNP® terms belonging to the Diagnosis/Outcome (DC) and Intervention (IC) axes were validated by the experts. Of these, 99.65% were found to be compatible with SNOMED CT. In addition, 97 new terms (30 Diagnoses/Outcomes and 67 Interventions) were validated and are currently awaiting approval by the International Council of Nurses. Of the newly proposed terms, 93.81% were compatible with SNOMED CT. Conclusions: The proposed Subset consists of 576 ICNP® terms, including 177 Diagnoses/Outcomes and 399 Interventions. Its implementation may support the adoption of electronic health records in neonatal and NICU settings and contribute to improving the quality and standardization of nursing care.