The soluble form of PD-L1 (sPD-L1) is related to a poor prognosis in various cancers. Comparisons of sPD-L1 and PD-L1 expressed on tumor cells in soft tissue tumor patients have not been reported. The purpose of this study was to analyze serum sPD-L1 and PD-L1 levels in soft tissue tumor patients. A total of 135 patients with primary soft tissue tumors were enrolled in this study. The sPD-L1 level was quantitatively measured by enzyme immunoassay, and PD-L1 expression on high grade sarcoma cells was analyzed immunohistologically. There were no significant differences in sPD-L1 levels between benign (48) and soft tissue sarcoma (STS) patients (87). In STS, the high sPD-L1 (>44.26 pg/mL) group had significantly lower metastasis-free survival (MS) and lower overall survival (OS) than the low sPD-L1 group (≤44.26 pg/mL) at 5 years using the log-rank test. On multivariate Cox proportional hazard analysis, the high sPD-L1 group had significant differences in MS and OS compared to the low sPD-L1 group. Between positive and negative immunostaining groups, recurrence-free survival (RS), MS, and OS were not significantly different. No correlation was found between immunostaining and sPD-L1 with the Kappa coefficient. The sPD-L1 concentration could predict future metastasis and prognosis in STS patients. High sPD-L1 in STS patients may be a target for treatment with checkpoint inhibitors.