The Keap1 (Kelch-like ECH-associated protein 1)-Nrf2 (nuclear factor erythroid 2-related factor 2)-ARE (antioxidant response element) pathway is the major defending mechanism against oxidative stresses, and directly disrupting the Keap1-Nrf2 protein-protein interaction (PPI) has been an attractive strategy to target oxidative stress-related diseases, including cardiovascular diseases. Here, we describe the design, synthesis, and structure-activity relationships (SARs) of indoline-based compounds as potent Keap1-Nrf2 PPI inhibitors. Comprehensive SAR analysis and thermodynamics-guided optimization identified 19a as the most potent inhibitor in this series, with an IC of 22 nM in a competitive fluorescence polarization assay. Further evaluation indicated the proper drug-like properties of 19a. Compound 19a dose-dependently upregulated genes and protein level of Nrf2 as well as its downstream markers and showed protective effects against lipopolysaccharide-induced injury in both H9c2 cardiac cells and mouse models. Collectively, we reported here a novel indoline-based Keap1-Nrf2 PPI inhibitor as a potential cardioprotective agent.