<p>This study systematically investigates the influence of indium (In) additions on the microstructure and comprehensive properties of Al–Cu–Mg alloys. In this paper, the influence of In additions on the microstructure of Al–Cu–Mg alloy was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Concurrently, the alloy’s mechanical and corrosion behaviors are quantitatively evaluated via Vickers hardness measurements, tensile testing, electrochemical polarization, exfoliation corrosion, and intergranular corrosion analyses. Results show that In additions significantly enhance both mechanical properties and corrosion resistance. Specifically, the 0.2 wt% In-containing alloy subjected to heat treatment (500 °C/1 h solid solution + 200 °C/4 h aging) exhibits remarkable improvements in properties: a 31.2% increase in hardness and a 25.8% enhancement in tensile strength compared to the In-free alloy. Electrochemical characterization reveals superior corrosion resistance in 0.2 wt% In-containing alloy, as evidenced by its exceptional charge transfer resistance (<i>R</i>_ct = 32,464 Ω cm²) and minimal corrosion current density (<i>i</i>_corr = 1.40 × 10⁻³ mA cm²), representing a substantial improvement over In-free alloy. TEM analysis further confirms that In additions promote the uniform precipitation of <i>θ′</i> strengthening phases (Al₂Cu) while refining their spatial distribution. These microstructural modifications are identified as the primary mechanism underlying the simultaneous enhancement of mechanical properties and corrosion resistance in In-modified Al–Cu–Mg alloys.</p>