<p>Expansive stiff clays are widely distributed around the world and typically exist in an unsaturated state in earthworks. Recent studies have highlighted the influence of suction on the small-strain shear modulus (<i>G</i>_max) of geomaterials and reported stiffness decay in response to environmental changes. However, the coupled effects of suction gradients and drying history on <i>G</i>_max, along with the underlying microstructural evolution, remain insufficiently understood. In this study, a series of bender element tests and mercury intrusion porosimetry tests were conducted on intact stiff clay samples subjected to two drying paths controlled by vapor equilibrium (VE) and air drying (AD) methods, respectively. The results showed that <i>G</i>_max initially increased and then decreased during the progressive drying. Such evolution trend of <i>G</i>_max under the VE method presented a delayed response compared to that observed under the AD method. This difference was attributed to the hysteretic evolution of pore size distribution due to differences in dehumidification intensity. By correlating the shrinkage behavior with stiffness evolution, it was found that <i>G</i>_max increased under both drying methods at the normal shrinkage stage, which can be directly attributed to a reduction in the void ratio. During the residual and zero shrinkage stages, <i>G</i>_max began to decline under the AD method, while it continued to increase under the VE conditions. This divergence could be explained by the effect of suction gradient on microstructure evolution: As the initial sample with suction of 0.35&#xa0;MPa was equilibrated to higher suctions (≥ 9.0&#xa0;MPa), the primary influence on stiffness shifts from microstructural densification (a decrease in intruded void ratio) to dominant pore family variations. Moreover, the observed stiffness deterioration was related to desiccation cracking and the degradation of cementation bonds.</p>