Background: Accurate preoperative localization of hyperfunctioning parathyroid glands is essential for minimally invasive surgery in primary hyperparathyroidism (PHPT). Although dual-phase 99mTc-MIBI planar scintigraphy is widely used, its diagnostic performance may be limited in small or ectopic lesions. Hybrid SPECT/CT imaging combines functional and anatomical information and may improve localization accuracy. The aim of this study is to evaluate the diagnostic contribution of SPECT/CT to dual-phase 99mTc-MIBI planar scintigraphy for preoperative localization in patients with PHPT. Methods: This retrospective study included 128 patients with biochemically confirmed PHPT who underwent dual-phase 99mTc-MIBI planar scintigraphy followed by delayed-phase SPECT/CT imaging before parathyroidectomy between January 2020 and May 2024. Imaging findings were compared with postoperative histopathological results. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy were calculated. The effects of lesion size, biochemical parameters, and ultrasonographic findings on imaging performance were also evaluated. Results: Histopathology confirmed parathyroid lesions in 122 of 128 patients (95.3%). The sensitivity, specificity, PPV, NPV, and accuracy of planar imaging were 57.4%, 66.7%, 97.2%, 7.1%, and 57.8%, respectively, whereas those of SPECT/CT were 92.6%, 66.7%, 98.3%, 30.8%, and 91.4%, respectively. SPECT/CT findings showed significant concordance with histopathology (p=0.001), while planar imaging did not (p=0.247). Among lesions smaller than 1 cm, sensitivities were 37.1% for planar imaging and 91.4% for SPECT/CT. SPECT/CT also provided superior anatomical localization, particularly in ectopic lesions and in patients with concomitant thyroid pathology. Conclusion: SPECT/CT demonstrated substantially higher diagnostic performance than dual-phase planar scintigraphy for preoperative localization in PHPT. Its incremental value was particularly evident in subcentimeter lesions, ectopic parathyroid adenomas, and patients with coexisting thyroid disease.