The sibling relationship and its potential impact on neurodevelopment and mental health are important areas of neuroscientific research. Validation of the tools assessing the quality of the sibling relationship would be the first essential step for conducting neurobiological and psychosocial studies related to the sibling relationship. However, to the best of our knowledge, no sibling relationship assessment tools have been empirically validated in Korean. We aimed to evaluate the psychometric properties of the Korean version of the Lifespan Sibling Relationship Scale (LSRS), which is one of the most commonly used self-report questionnaires to assess the quality of the sibling relationship. A total of 109 adults completed a series of self-report questionnaires including the LSRS, the mental health subscale of the Medical Outcomes Study-Short Form 36 version 2 (SF36v2), the Satisfaction with Life Scale (SLS), and the Marlowe-Crowne Social Desirability Scale (MC-SDS). The internal consistency, subscale intercorrelations, one-week test-retest reliability, convergent validity, divergent validity, and the construct validity were assessed. All six subscale scores and the total score of the LSRS demonstrated good internal consistency (Cronbach's α=0.85-0.94) and good test-retest reliability (intraclass correlation coefficient=0.77-0.92). Correlations of the LSRS with the SF36v2 mental health score (r=0.32, p=0.01) and with the SLS (r=0.27, p=0.04) supported the good convergent validity. The divergent validity was shown by the non-significant correlation of the LSRS with the MC-SDS (r=0.15, p=0.26). Two factors were extracted through factor analysis, which explained 78.63% of the total variance. The three Adult subscales loaded on the first factor and the three Child subscales loaded on the second factor. Results suggest that the Korean version of the LSRS is a reliable and valid tool for examining the sibling relationship.