Objective: To develop an operational definition of the “diagnostic reasoning” concept in nursing. Materials and methods: This concept analysis is based on Walker and Avant’s model. All the referential steps were followed: concept selection, determining the objectives and proposals for concept analysis, identifying the possible uses of the concept, determining the defining attributes, identifying a model case, identifying the contrary case, and identifying the precedents and consequences of the concept. Results: A sample of 20 studies was selected, identifying the following attributes: specialized thinking, cognitive skills, existing knowledge, intellectual competence; hypothesis-oriented reasoning; precedents: clinical assessment, practical experience, data interpretations, intuition, diagnostic judgment, information processing, clinical reasoning; consequences: improvement of nursing care, communication, planning interventions, addressing the identified problem, achieving effective results, autonomous actions, and quality of patient records. Conclusions: The study contributes to understanding the operational definition of the diagnostic reasoning concept in nursing by identifying the core attributes, precedents, and consequences.