Background: Sarcoma cells are most immunogenic at the onset of cancer when the immune system can recognize and destroy them. Hence, immune checkpoint inhibitors would be most effective when given as first line therapy. Objectives: (1) To investigate the maximum tolerated dose of trabectedin, an alkylating agent, when given sequentially with ipilimumab, a CTLA4 inhibitor, and nivolumab, a PD-1 inhibitor, in advanced STS, (2) To investigate the objective response rate (ORR), progression free survival (PFS) and overall survival (OS) , and (3) To correlate PFS with PD-L1 and other biomarker expression in patients’ tumors. Methods: Forty patients ≥18 years of age with advanced STS will be enrolled. This is a phase 1/2 study using a defined dose of ipilimumab (1 mg/kg i.v. q 12 weeks), nivolumab (3 mg/kg i.v. q 2 weeks), and escalating doses of trabectedin (1.0, 1.3, 1.5 mg/m² i.v. q 3 weeks). I. Dose Escalation Phase 1 (previously treated patients): The study will employ the standard “cohort of three” design. The maximum tolerated dose is defined as the highest safely tolerated dose, where not more than one patient experienced DLT, with the next higher dose level having at least two patients who experienced DLT. II. Expansion Phase 2 (previously untreated patients): An additional 22-28 patients will receive trabectedin at the MTD and defined doses of ipilimumab and nivolumab to assess overall safety and potential efficacy in a greater number of patients. Patients may continue treatment until significant disease progression or unacceptable toxicity occurs. Statistical Considerations: NIH CTCAE v4.03 and RECIST v1.1 will be used. Categorical variables will be summarized by the n and percent in each category. Point estimates for efficacy endpoint incidences will be accompanied by a 2-sided 95% exact binomial CI. Time to event endpoints will be summarized descriptively using the KM method. The analyses of all study objectives will be descriptive and hypothesis generating, for planning Phase 2/3 studies. Clinical trial information: NCT 03138161.