Background This study aimed to assess the independent prognostic value of tumor size compared with other clinical and pathologic features of primary invasive cutaneous melanoma (CM). Methods This study included 28,593 patients with primary invasive CM in Surveillance, Epidemiology, and End Results Program database diagnosed from 2004 through 2016. Tumor size was divided into five subgroups (<= 6, 7-12, 13-30, 31-42, and >42 mm). The primary endpoint was melanoma-specific survival (MSS). Results The relationship between tumor size and survival was piecewise. After adjusting for age, sex, primary site, histopathologic cell type, Breslow thickness, ulceration, mitotic rate, regional metastasis, and distant metastasis, the hazard ratio (HR) of MSS increased with increasing tumor size until a peak at 31-42 mm (HRs, 1.33, 1.59, 2.41, respectively; all P < .0001), and then decreased when tumor size was larger than 42 mm using tumor size <= 6 mm as the reference (HR, 2.11; 95% confidence interval [CI], 1.84 -2.42; P < .0001). This pattern mostly remained after stratification by T subcategories from T1 to T4 in localized primary CM except that tumor size >42 mm subgroup had the shortest MSS in T4. In addition, tumor size with a cutoff value of 12 mm showed stronger prognostic value for MSS (HR, 2.32; 95% CI, 1.80-2.98; P < .0001) than Breslow thickness and mitotic rate in primary CM with T1N0M0. Conclusions Tumor size was an important independent prognostic factor for MSS in patients with primary invasive CM. Tumor size larger than 30 mm would provide additional and important prognostic information in each T subcategory of localized CM. Furthermore, tumor size with a cutoff value of 12 mm has great potential in improving the accuracy of melanoma T1 substaging.