Context: Thyroid incidentaloma diagnosed by 2-[18F]-fluoro-2-deoxy-d-glucose positron emission tomography/computed tomography (F-18-FDG-PET/CT) is defined as a thyroid uptake incidentally and newly detected in a patient studied for nonthyroid purpose. In this review, we have comprehensively analyzed the diagnostic and clinical significance of F-18-FDG-PET/CT thyroid incidentalomas revealed during studies performed for an unrelated and nonthyroid purpose. Evidence Acquisition: A comprehensive literature research of the PubMed/MEDLINE databases was conducted to find relevant published articles about the F-18-FDG-PET or F-18-FDG-PET/CT thyroid incidentalomas. Evidence Synthesis: All studies considered in this review have investigated a very large number of patients, achieving overall about 147,505 units. The pooled incidence of thyroid incidentalomas detected by F-18-FDG-PET or PET/CT was 2.46% (95% confidence interval, 1.68–3.39%). The malignancy ratio was 34.6% (95% confidence interval, 29.3–40.2%). Conclusion: F-18-FDG-PET/CT thyroid incidentaloma is a relevant clinical finding; diffuse uptakes and most focal uptakes are commonly caused by benign diseases, whereas about one third of focal uptakes are malignant; the most frequent malignant histological type responsible for F-18-FDG-PET/CT thyroid incidentaloma is papillary thyroid carcinoma.