© 2020 Japan College of Rheumatology.Objective: To investigate the validation and reliability of Istanbul Low Back Pain Disability Index (ILBPDI) in axial spondyloarthritis (Ax-SpA). Methods: Patients with Ax-SpA according to The Assessment of SpondyloArthritis International Society criteria were recruited. The validation was assessed by face, content, and construct (convergent and divergent) validities, whereas the reliability was assessed by internal consistency and test–retest reliability. Factor analysis was performed. Convergent validity was assessed by correlations of ILBPDI with functional parameters (The Bath Ankylosing Spondylitis Functional Index, The Dougados Functional Index, and The Health Assessment Questionnaire). Divergent validity was assessed by correlations of ILBPDI with non-functional parameters. Results: Two hundred forty patients were recruited. Cognitive debriefing showed ILBPDI to be clear, relevant, and comprehensive. Cronbach’s alpha coefficient was 0.953. The test–retest reliability was good with the intraclass correlation coefficient of 0.870. ILBPDI was represented by three-factor groups of activity: axial bending, sitting/rest, and standing activities. ILBPDI had good correlations with the functional parameters (rho changes between 0.809 and 0.580), and it had poor or non-significant correlations with the non-functional parameters (absolute rho changes between 0.669 and 0.001). Conclusion: ILBPDI is a practical, accurate, and non-time-consuming scale which is valid and reliable to evaluate the functional disability in patients with Ax-SpA.