Purpose We aimed to evaluate F-18-fluorodeoxyglucose positron emission tomography/computerized tomography (F-18-FDG PET/CT) derived parameters in primary and metastatic renal tumors. Methods A total of 47 patients with renal malignant tumors who underwent assessment in our department between December 2015 and March 2022 were enrolled in this study. The diagnoses were confirmed by biopsy or surgical resection plus histological study. Clinical and imaging information were collected. Comparisons among groups were performed with the independent sample t test or Mann-Whitney U test, as appropriate. Logistic regression and ROC curve analysis were used to calculate and compare the diagnostic value of F-18-FDG PET/CT in primary renal malignant tumors and metastatic renal malignant tumors. Results The maximum diameter and metabolic tumor volume (MTV) of primary renal tumors were larger than those of metastatic renal tumors, which had a smaller maximum diameter and MTV. Primary renal tumors are smaller than metastatic renal tumors in terms of maximum standardized uptake value (SUVmax), mean standardized uptake value (SUVmean), peak standardized uptake value (SUVpeak), tumor-to-background ratio (TBR), and tumor-to-liver ratio (TLR). The SUVmax of clear cell primary renal carcinoma was lower than the SUVmax of clear cell primary renal carcinoma. Metastatic renal tumors associated with squamous lung cancer had a higher SUVmax than those associated with non-squamous lung cancer. Conclusion F-18-FDG PET/CT may be a useful imaging modality for detecting primary renal tumors and metastatic renal tumors and may provide valuable information for the differential diagnosis of primary renal tumors and metastatic renal tumors. The SUV, TBR, TLR, maximum diameter, and MTV of a renal mass are potential predictive factors to distinguish primary and metastatic renal tumors. In addition, F-18-FDG PET/CT revealed differences in metabolic activity based on the histopathological type of renal tumors, which may help to individualize treatment.