The National Cancer Institute Molecular Analysis for Therapy Choice (NCI-MATCH) trial, the largest national precision oncology study to date (> 1,100 sites) of patients with relapsed or refractory malignancies, assigned patients to targeted therapy in parallel phase II studies based on tumor molecular alterations. The anti-programmed death receptor 1 inhibitor nivolumab previously showed activity in mismatch repair (MMR)-deficient colon cancer. We hypothesized that nivolumab would have activity in patients with MMR-deficient, noncolorectal tumors. Eligible patients with relapsed or refractory tumors, good end-organ function, and Eastern Cooperative Oncology Group performance status of ≤ 1 underwent tumor biopsy for centralized screening of molecular alterations. MMR deficiency was defined by complete loss of nuclear expression of or MMR gene products by immunohistochemistry (IHC). Patients with MMR-deficient colorectal cancer were excluded. Nivolumab, 3 mg/kg every 2 weeks (28-day cycles) and 480 mg every 4 weeks after cycle 4, was administered intravenously. Disease reassessment was performed every 2 cycles. The primary end point was RECIST 1.1 objective response rate (ORR). Two percent of 4,902 screened patients had an MMR-deficient cancer by IHC. Forty-two evaluable patients were enrolled, with a median age of 60 years and a median of 3 prior therapies. The most common histologies were endometrioid endometrial adenocarcinoma (n = 13), prostate adenocarcinoma (n = 5), and uterine carcinosarcoma (n = 4). ORR was 36% (15 of 42 patients). An additional 21% of patients had stable disease. The estimated 6-, 12-, and 18-month progression-free survival rates were 51.3% (90% CI, 38.2% to 64.5%), 46.2% (90% CI, 33.1% to 59.3%), and 31.4% (90% CI, 18.7% to 44.2%), respectively. Median overall survival was 17.3 months. Toxicity was predominantly low grade. A variety of refractory cancers (2.0% of those screened) had MMR deficiency as defined in NCI-MATCH. Nivolumab has promising activity in MMR-deficient noncolorectal cancers of a wide variety of histopathologic types.