Abstract Purpose: Immune checkpoint inhibitors show high response rates and durable clinical benefit in microsatellite instability–high/mismatch repair–deficient (MSI-H/dMMR) tumors. However, 50%–60% do not respond to single-agent anti–programmed death-1/programmed death ligand 1 (PD-1/PD-L1) antibodies, and approximately 50% of responders relapse within 6–12 months. This phase Ib trial evaluated safety and antitumor activity of anti–PD-L1 antibody LY3300054 monotherapy or in combination with anti–TIM-3 antibody LY3321367 in patients with MSI-H/dMMR advanced solid tumors. Patients and Methods: Eligible patients ≥18 years without prior anti–PD-1/PD-L1 therapy received LY3300054 monotherapy (N = 40) or combination (N = 20); patients with PD-1/PD-L1 inhibitor–resistant/refractory tumors received the combination (N = 22). LY3300054 (700 mg) and anti–TIM-3 antibody (cycles 1–2: 1,200 mg, cycle 3 onward: 600 mg) were administered intravenously every 2 weeks. Primary endpoints were safety and tolerability. Results: Eighty-two patients were enrolled. Most had colorectal (n = 39, 47.6%) or endometrial (n = 14, 17.1%) tumors. More than 70% of patients in the PD-1/PD-L1 inhibitor–resistant/refractory combination cohort had received ≥3 treatment lines. Treatment-related adverse events (TRAE) occurred in 22 patients (55.0%) receiving monotherapy, 13 (65.0%) in the PD-1/PD-L1 inhibitor–naïve combination cohort, and 6 (27.3%) in the PD-1/PD-L1 inhibitor–resistant/refractory combination cohort. A total of 2 patients (5.0%) receiving monotherapy and 3 (7.1%) receiving the combination experienced grade ≥3 TRAEs. Objective responses occurred in 13 patients (32.5%) with monotherapy, 9 (45.0%) in the PD-1/PD-L1 inhibitor–naïve combination cohort, and 1 patient (4.5%) in the PD-1/PD-L1 inhibitor–resistant/refractory combination cohort. Conclusions: LY3300054 monotherapy and combined LY3300054/anti–TIM-3 had manageable safety profiles. Both regimens showed promising clinical activity against PD-1/PD-L1 inhibitor–naïve MSI-H/dMMR tumors. The combination had limited clinical benefit in patients with PD-1/PD-L1 inhibitor–resistant/refractory MSI-H/dMMR tumors.