Limited evidence is available in the literature regarding the fate of the unfused structural thoracic curve following selective thoracolumbar-lumbar curve fusion (SLF) in Lenke 6C adolescent idiopathic scoliosis (AIS) patients. Therefore, we compared the outcomes of SLF between Lenke 6C and 5C AIS patients. We retrospectively reviewed 31 patients Lenke 5C (n = 18) and Lenke 6C (n = 13) AIS patients who underwent SLF at a single institution. Multiple radiological parameters were measured using whole-spine radiographs taken before and after surgery and at the last follow-up visit. SRS-22 at the final follow-up was obtained for clinical assessment. A total of 31 patients with a mean age of 14.6 years at operation who were followed for a mean of 6.4 years were included in this study. The Cobb angle of the unfused thoracic curve was spontaneously corrected immediately following SLF and increased slightly but not significantly at the final follow-up in both groups (Lenke 5C: pre 33.0°, post 14.4°, final 19.4°, Lenke 6C: pre 46.1°, post 31.7°, final 34.2°). At every time point, the thoracic Cobb angle was significantly larger in the Lenke 6C. SRS-22 score at the final follow-up, including the self-image domain, did not differ between the two groups. In this study, SLF for Lenke 6C AIS achieved a significant spontaneous correction of the unfused thoracic curve and yielded a comparable SRS-22 result at the final follow-up to that of Lenke 5C. Our findings suggest that SLF is a viable treatment option for Lenke 6C AIS.