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Introduction: This is the first study to analyze the associations between the radiological severity of hip involvement with clinical characteristics and sagittal spinopelvic balance in patients with ankylosing spondylitis (AS). Method: We evaluated 182 patients with AS who were referred to outpatient clinics. Patient demographic data and clinical and radiographic parameters were collected. Patients were divided into three groups based on the Bath Ankylosing Spondylitis Radiology Hip Index. Clinical characteristics and spinopelvic parameters acquired by a low-dose biplanar imaging system were evaluated among these groups. Results: Patients with more severe hip involvement were older and had longer disease duration and diagnostic delay, with lower Harris Hip Score (p < 0.001) and 12-item Short Form Health Survey Physical Component Score (p < 0.001) and higher Bath Ankylosing Spondylitis Disease Activity Index (p = 0.030) and Functional Index (p < 0.001). Patients with more severe hip involvement had significantly higher sacroiliac grade (p < 0.001) and higher modified Stoke Ankylosing Spondylitis Spinal Score (p < 0.001). Patients with moderate and severe hip involvement had similar lumbar lordosis and spino-sacral angle, whereas patients with severe hip involvement had lower pelvic tilt, pelvic femoral angle, higher sacral slope, and sagittal vertical axis. Conclusions: The severity of hip involvement is associated with physical function and is not consistent with the severity of spinal involvement. Severe hip involvement impairs the ability to retrovert the pelvis to accommodate the sagittal deformity, and spinopelvic parameters should be concretely evaluated in preoperative counseling of patients with AS waiting for total hip arthroplasty. Key Points • The severity of hip involvement in patients with AS is associated with physical function. • Severe hip involvement impairs the ability to retrovert the pelvis to accommodate the sagittal deformity. [ABSTRACT FROM AUTHOR]