The relationship between spinal sagittal alignment and pelvic parameters is well known in adolescent idiopathic scoliosis. However, few studies have reported the sagittal spinopelvic relationship after selective posterior fusion of thoracolumbar/lumbar (TL/L) curves. We evaluated the relationship between spinal sagittal alignment and the pelvis, and analyzed how the pelvic sagittal state is adjusted in Lenke type 5C patients. We conducted a retrospective study of 36 patients with Lenke type 5C curves who received selective posterior TL/L curve fusion. Coronal and spinopelvic sagittal parameters were pre and postoperatively compared. Pearson coefficients were used to analyze the correlation between all spinopelvic sagittal parameters before and after surgery. We also evaluated 3 pelvic morphologies (anteverted, normal, and retroverted) before and after surgery. Preoperatively, the mean pelvic incidence was 46.0°, with a pelvic tilt and sacral slope (SS) of 8.2° and 37.8°, respectively, and 25% (9/36) of patients had an anteverted pelvis, whereas the other 75% had a normal pelvis. Postoperatively, 42% (15/36) of patients had a retroverted pelvis, 53% (19/36) had a normal pelvis, and 2 patients had an anteverted pelvis. Logistic regression analyses yielded 2 factors that were significantly associated with the risk for a postoperative unrecovered anteverted pelvis, including increased lumbar lordosis (LL) (odds ratio [OR] 4.8, P = 0.029) and increased SS (OR 5.6, P = 0.018). Four factors were significantly associated with the risk of a postoperative newly anteverted pelvis, including LL at the final follow-up (OR 6.9, P = 0.009), increased LL (OR 8.9, P = 0.003), LL below fusion (OR 9.4, P = 0.002), and increased SS (OR 11.5, P = 0.001). The pelvic state may be adjusted after selective posterior TL/L curve fusion in Lenke 5C adolescent idiopathic scoliosis patients. It is difficult to improve an anteverted pelvis in patients who have an LL that is increased by more than 11.6° or an SS that is increased by more than 4.7° after surgery. The anteverted pelvic state will generally occur with posterior correction surgery for patients with an LL that is greater than 63.7°, or an LL or SS that is respectively increased by more than 17.6° or 9.0° postoperatively.