Surgical stabilization of high-grade lumbosacral spondylolisthesis is clinically challenging, and the success of deformity reduction and fusion varies. The authors describe a patient with Grade III spondylolisthesis at L5-S1. Partial reduction was achieved and fusion involved pedicle screw fixation and a posterior transvertebral interbody cage. This patient had developed progressive spondylolisthesis after decompression and posterolateral fusion for L5-S1 spondylolisthesis failed. Clinical and early radiographic results were excellent. Transsacral cage fixation can be considered a viable option to buttress the region in which high-grade L5-S1 spondylolisthesis has been reduced. The cage provides substrate for interbody arthrodesis and acts as a biomechanical stabilizer that helps prevent pedicle screw failure.