En bloc resection of iliosacral sarcomas is a surgical challenge. There are substantial risks of inadequate margins, local recurrence, and nerve root loss when pelvic sarcomas involve sacral root canals and foramina. The decancellation technique uses principles similar to transpedicle decancellation in spinal deformity correction to perform the sacral osteotomy in iliosacral tumor resection. The technique aims at improving sacral margins and minimizing loss of neural function. We performed a decancellation osteotomy in five patients with sarcomas requiring difficult oblique or sagittal sacral osteotomies and selective root sacrifice. Through laminectomy and without anterior exposure, a precise full-thickness osteotomy of the sacrum was performed without major technique-related morbidities or complications. This was followed by formal pelvic resection and reconstruction. Surgical margins were adequate in all patients and all tumor-free nerve roots were preserved.