BackgroundIn recent years, a growing body of literature on informal caregivers’ health and well-being has emerged, highlighting the need for a tool to measure their self-care practices. ObjectiveThe aim of the study was to develop a theory-based instrument measuring the self-care behaviors of informal caregivers and test its psychometric properties. DesignThe initial phase of instrument development entailed a detailed, six-step process (conceptualization, theoretical adaptation, back-translation and cultural adaptation, cognitive interviewing, item enhancement, and content validity), followed by formal psychometric testing (participant engagement, validity, internal consistency reliability, test-retest reliability). SettingStudy conducted at a hospital located in Tehran, Iran. ParticipantsA sample of 234 informal caregivers of cancer outpatients receiving treatment in oncology wards was enrolled. Caregivers had to be 18 years or older, recognized as the primary caregiver by the patient, and willing to provide informed consent. MethodsThe Self-Care of Informal Caregivers Inventory items comprise three dimensions: self-care maintenance (11 items), self-care monitoring (7 items), and self-care management (9 items), which achieved a content validity index rating of 100 % in a panel of experts. Data were collected from caregivers during routine clinic visits. Construct validity was verified through exploratory structural equation modelling and reliability was verified using Cronbach's α and multidimensional model-based reliability. Test-retest reliability was evaluated using the Intraclass Correlation Coefficient. ResultsAll three dimensions showed good model fit indices (self-care maintenance: Comparative Fit Index = 1.00, Tucker-Lewis Index = 0.99, Root Mean Square Error of Approximation = 0.044; self-care monitoring: Comparative Fit Index = 1.00, Tucker-Lewis Index = 1.00, Root Mean Square Error of Approximation = 0.027; self-care management: Comparative Fit Index = 0.99, Tucker-Lewis Index = 0.99, Root Mean Square Error of Approximation = 0.048) and Cronbach's α of 0.88, 0.88, and 0.91, respectively. The overall multidimensional model-based reliability was 0.93. The Intraclass Correlation Coefficient values for the three dimensions were 0.94, 0.60, and 0.51, respectively. ConclusionPreliminary testing provides support for use of the Self-Care of Informal Caregivers Inventory in research. Using this theory-based instrument to assess the self-care practices of informal caregivers can assist in identifying topics to discuss and opportunities for guidance. Tweetable AbstractThe Self-Care of Informal Caregivers Inventory: A validated tool for informal caregivers is useful for research. #Informalcaregivers #SelfCare