Aim: Life changes due to the sudden onset of acquired brain injury (ABI) are drastic personal and social changes that require adaptation and are also an important indicator of the quality of life of family caregivers. However, there are no instruments for evaluating life change adaptation among family caregivers of individuals with acquired brain injury. This study aimed to develop the Life Change Adaptation Scale (LCAS) for family caregivers of individuals with ABI and examine its reliability and validity. Methods: A cross-sectional study was conducted using a self-reported questionnaire. A total of 1622 family caregivers of individuals with ABI who belonged to 82 associations for families of individuals with ABI were selected as eligible participants. The construct validity was evaluated using a confirmatory factor analysis. Internal consistency was calculated using Cronbach's alpha. The K6 was also administered to assess the criterion-related validity of the LCAS. Results: In total, 339 valid responses were received. The confirmatory factor analysis identified eight items from two domains, "Changes in the appraisal of caregiving resources" and "Changes in the health belief as a caregiver" (goodness of fit index = 0.963, adjusted goodness of fit index = 0.926, comparative fit index = 0.986, root mean square error of approximation = 0.043.) Cronbach's alpha was 0.84. The LCAS was negatively correlated with the K6 (r = -0.504; P<0.001). Conclusions: The LCAS is a brief, easy-to-administer instrument that is reliable and valid for family caregivers of individuals with ABI. This study contributes to the assessment and identification by family caregivers of individuals with ABI who require aid in adapting to life changes. Further research should be undertaken to verify the predictive value in a longitudinal study and to attempt to apply the LCAS to assess a broader range of subjects in a wider range of settings.