The efficacy of immune checkpoint blockade in gestational trophoblastic neoplasia (GTN) remains uncertain. We report the results of the GTN cohort of SWOG S1609 dual anti-CTLA-4 and anti-PD-1 blockade in rare tumors (DART). This prospective, open-label phase II trial evaluated ipilimumab plus nivolumab across multiple rare tumor cohorts, including GTN. Eligible patients received nivolumab 240 mg, i.v. every 2 weeks and ipilimumab 1 mg/kg i.v. every 6 weeks. The primary endpoint was overall response rate ORR; complete response (CR) + partial response (PR) by quantitative serum beta human chorionic gonadotropin (β-hCG); secondary endpoints included progression-free survival (PFS), overall survival (OS), and toxicity. Four patients with refractory GTN enrolled and received therapy. At 11 months of ongoing follow-up, 3 of 4 patients responded ORR = 75% (CR, 25%, n = 1, tumor mutation burden = 1 mutation/megabase; PD-L1 tumor proportion score = 50%); PR, 50%, n = 2). Responders included malignant gestational trophoblastic neoplasm (n = 1, CR, PFS 11+ months) and choriocarcinoma (n = 2, both PRs, PFS 10+ and 6+ months). One patient with epithelioid trophoblastic tumor experienced disease progression. The 6-month PFS was 75% 95% confidence interval (CI), 43%-100%, and the median PFS was not reached (range, 35-339+ days); all 4 patients were alive at last follow-up. Two patients experienced grade 3 immune-related toxicity (arthralgia and colitis); there were no grade ≥4 events. Ipilimumab plus nivolumab demonstrated efficacy in chemotherapy-refractory GTN, an ultra-rare cancer affecting young women. Three of 4 patients achieved ongoing objective responses with a reasonable safety profile at 6-11+ months.