Adult spinal deformity (ASD) surgery has increased in prevalence in recent years, with the majority of patients aged 65 or older. While perioperative complications are reported to occur in approximately 40% of cases, venous thromboembolism (VTE), which can be life-threatening, remains understudied in this specific population. Current guidelines classify spinal surgery broadly as "moderate risk" for VTE without distinguishing between procedure types, raising the question of whether this risk stratification is adequate for the more invasive ASD surgery. We prospectively applied a standardized VTE screening protocol to 95 patients who underwent ASD surgery and 103 patients who underwent posterior spinal fusion (PSF) at our institution between January 2021 and March 2025. Serial D-dimer measurements were obtained on postoperative days 1, 3, 7, and 14. Patients meeting predefined elevation criteria underwent lower extremity venous ultrasonography, followed by contrast-enhanced CT when thrombosis was identified. Sarcopenia was assessed using the Psoas Muscle Index (PMI), calculated from cross-sectional MRI at the L3 level. Deep vein thrombosis (DVT) was detected in 44% of ASD cases and 13% of PSF cases. Pulmonary embolism (PE) occurred in 14% of ASD patients and in 3% of PSF patients, with five cases being symptomatic. Notably, among patients with distal DVT, PE was concurrently identified in 11%, despite current VTE guidelines not recommending routine CT pulmonary angiography for isolated distal thrombosis. Univariate analysis followed by logistic regression identified operative time as the only independent risk factor for VTE in ASD cases, with ROC analysis yielding a cutoff of 398 minutes (AUC 0.836). Sarcopenia as assessed by PMI was not identified as an independent risk factor. These findings suggest that ASD surgery carries a substantially higher VTE risk than currently recognized under existing guidelines. Prophylactic anticoagulation, routinely used in total knee and hip arthroplasty, warrants serious consideration for ASD surgery. Furthermore, PE evaluation should be considered even in postoperative patients presenting with distal-only DVT.