The accurate diagnosis of soft tissue tumors may be difficult. Simple clinical characteristics or laboratory data that can predict tumor malignancy can be useful tools for diagnosing soft tissue tumors. Between 2003 and 2018, 588 patients with primary soft tissue tumors were retrospectively reviewed. Their clinical characteristics and laboratory data were evaluated to determine their association with the diagnosis of benign, intermediate, or malignant tumor. Multivariable analysis revealed that tumor size ≥ 5.6 cm (odds ratio (OR), 6.15; p < 0.001), white blood cell (WBC) count ≥ 5700/µL (OR, 2.49; p = 0.002), hemoglobin (Hb) count ≤ 12.4 g/dL (OR, 2.56; p = 0.004), C-reactive protein (CRP) level ≥ 0.17 mg/dL (OR, 2.64; p < 0.001), and lactate dehydrogenase (LDH) level ≥ 240 IU/L (OR, 4.94; p < 0.001) were significant predictive factors for sarcoma. The sensitivity and specificity in the presence of three or more predictive factors for detecting malignant tumors were 0.58 and 0.90 respectively, and it was an appropriate threshold with the maximum Youden’s index of 0.49. Simple clinical and laboratory data were useful tools for predicting whether the tumor is malignant. Patients with soft tissue tumors that meet any three or more predictive factors should be referred to a specialist.