<p>Biochar-modified clay has garnered growing interest in geotechnical engineering, yet existing research has predominantly focused on swelling–shrinkage behavior, strength, and hydraulic conductivity, with comparatively little attention given to stiffness evolution. This study systematically investigates the effects of biochar particle size (&lt; 0.075 mm, 0.075–0.425 mm, and 0.425–2 mm) and mass content (0%, 5%, 10%, and 15%) on the small-strain stiffness characteristics of amended expansive clay under varying effective consolidation stresses. Microstructural changes were examined using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). Results indicate that finer biochar particles markedly enhance initial stiffness but also accelerate its degradation with strain, whereas coarser particles attenuate stiffness reduction more slowly. Increasing biochar content generally promotes more brittle behavior. A modified Hardin–Drnevich model accurately captures the maximum shear modulus and its decay behavior across all biochar contents, with prediction errors within 15%. A logarithmic relationship between dimensionless confining pressure and reference shear strain reveals that higher confining pressures mitigate the influence of biochar content on normalized stiffness attenuation. Microstructural analyses show that fine biochar particles fill interaggregate pores, leading to a pronounced stiffening effect, while medium and coarse particles bond with soil particles through surface adsorption, enhancing stiffness via interparticle adhesion. These findings offer practical guidance for sustainable soil improvement strategies in mountain and slope environments, where stiffness-dependent deformation is critical.</p>