Purpose <p>The present study aimed to analyze the preoperative degeneration of the sacroiliac joint (SIJ) and its involvement in postoperative pelvic incidence (PI) change.</p> Methods <p>Data from consecutive patients who underwent corrective surgery for adult spinal deformity (ASD) were included in this study. Pre- and early postoperative full-standing radiographs and preoperative pelvic computed tomography (CT) images were examined. Spinopelvic parameters were measured from full-standing radiographs, whereas SIJ degeneration was evaluated from pelvic CT images. We then compared preoperative and postoperative spinopelvic parameters, focusing on PI change, and investigated whether spinopelvic parameters and SIJ degeneration affected PI change.</p> Results <p>Data from 122 patients (mean age, 71.8 years) were analyzed, with the mean number of fixed vertebrae being 10.2. Postoperative PI decreased significantly by 3.2° (from 50.8° to 47.6°; <i>P</i> &lt; 0.0001). After confirming the presence of SIJ degeneration based on preoperative CT, we found that 39, 38, 49, 98, and 54 patients had osteophytes, sclerosis, bridging bone, articular gas, and SIJ subluxation, respectively. No significant association was observed between sclerosis, osteophyte formation, bony bridging, or articular gas and postoperative PI change, although a significantly greater decrease in postoperative PI was observed among those with SIJ subluxation (<i>P</i> = 0.0035). PI change only correlated with preoperative PI (<i>P</i> &lt; 0.001, <i>R</i> = − 0.56).</p> Conclusion <p>PI decreased significantly after ASD surgery. We found that several patients showed SIJ subluxation before ASD surgery. PI change after ASD surgery was significantly associated with preoperative PI value and SIJ subluxation.</p>