PURPOSE: This study aimed to investigate the relationship of preoperative hemoglobin levels as an independent prognostic factor for hospital and intensive care unit (ICU) length of stay (LOS) in patients undergoing surgery for adult spinal deformity (ASD), with the intent of determining whether there exists a correlation and enhancing patient preoperative optimization protocols. METHODS: The authors reviewed consecutive patients who underwent elective thoracolumbosacral posterior spinal fusion (PSF) involving six or more vertebrae for ASD from January 1, 2013, to December 13, 2021, with a minimum follow-up period of two years. This study primarily investigated the association of preoperative hemoglobin levels with hospital and ICU LOS. To analyze the data, both unadjusted and adjusted generalized linear models (GLM), incorporating cubic splines for non-linear variables, were applied. RESULTS: A total of 598 patients were included. GLMs for hospital and ICU LOS demonstrated nonlinear relationships with preoperative hemoglobin levels. Specifically, hospital LOS decreased with increasing preoperative hemoglobin until a significance threshold of 13.5 g/dl. Similarly, ICU LOS significantly decreased with increasing preoperative hemoglobin until 13.0 g/dl. Lower preoperative hemoglobin was associated with more perioperative transfusions, less likely discharge to home, and greater risk of reoperation. CONCLUSIONS: Preoperative anemia is an independent non-linear risk factor that significantly affects LOS, disposition, and outcomes after surgery for ASD. These findings advocate for a systemic preoperative approach and highlight the need for future research to improve postoperative outcomes and reduce hospital resource utilization. LEVEL OF EVIDENCE: IV.