Background: Sarcoma cells are most immunogenic earlier in the disease course and before treatment when the immune system can recognize and destroy them. Hypothesis: Immune checkpoint inhibitors would be most effective when given to previously untreated patients with metastatic soft tissue sarcoma. Methods: Eligible patients for this Phase 2 study are previously untreated patients ≥ 18 years of age with unresectable or metastatic soft tissue sarcoma, with measurable disease by RECIST v1.1. Immune checkpoint inhibitors Ipilimumab (I) and Nivolumab (N) were given with Trabectedin (T), a marine derived alkaloid with defined doses of I (1 mg/kg i.v. q 12 weeks), N (3 mg/kg i.v. q 2 weeks), and T (1.2 mg/m2 i.v. q 3 weeks). Primary endpoints: (1) Objective response rate by RECIST v1.1 via CT scan or MRI, (2) Progression-free survival (PFS): from first day of treatment to disease progression or death due to any cause; otherwise, it is censored at the time of last follow-up, and (3) Overall survival: from first day of treatment to death due to any cause; otherwise, it is censored at the time of last follow-up. Results: There were eighty-two evaluable subjects, having completed the first cycle of I, N, and T and have had a CT or MRI scan at the 6-week follow-up period. Best Overall Response by RECIST v1.1 = 7 CR (2 surgical CR), 9 PR, 54 SD, and 12 PD. Disease control rate was 85.4%. The median PFS was >6.4 (range: 0-32) months; 6-month PFS rate: 57.3%. The median OS was >12.0 (0-38) months; 6-month OS rate: 78.8%. Safety analysis: The most common Grade 3 TRAEs include increased ALT (26), anemia (11), increased AST (9), and fatigue (8). Common Grade 4 TRAEs include thrombocytopenia (2), increased AST (2), increased ALT (2), and increased CPK (2). There was one Grade 5 TRAE of rhabdomyolysis (1). Conclusions: Taken together, these results suggest that first-line combinatorial therapy with I, N, and T are (1) synergistic, and (2) may be equal or superior to, and safer than, standard first line therapy for advanced/metastatic soft tissue sarcoma. Clinical trial information: NCT03138161