OBJECTIVES/HYPOTHESIS The correlation between incidental parathyroidectomy (IP) during thyroidectomy and postoperative hypocalcemia remains controversial. Our aim was to investigate the incidence of IP, risk factors, and impact on patient outcomes. STUDY DESIGN Retrospective cohort study. METHODS This was a retrospective observational study including 3,186 consecutive patients who underwent thyroidectomy between January 2007 and December 2014. The patients were divided into two groups: the IP group and the non-IP. Numerous clinical parameters were collected and analyzed. RESULTS The overall incidence of incidentally excised parathyroid glands during thyroidectomy was 6.4%. Patients with IP had significantly higher incidences of postoperative hypocalcemia and hypoparathyroidism than those without IP (P < 0.001). Intrathyroidal parathyroid glands presented only 2.2% of all removed parathyroid glands. Total thyroidectomy, central compartment lymph node dissection, and reoperation were independent risk factors for IP. CONCLUSION Incidental parathyroidectomy during thyroidectomy is associated with the increased likelihood of postoperative hypocalcemia. All independent risk factors examined in the study for IP are surgery-related. Surgeons should perform meticulous dissection with the intention of avoiding IP and resultant hypocalcemia. LEVEL OF EVIDENCE 4. Laryngoscope, 127:2194-2200, 2017.