The small-strain shear stiffness of compacted Barcelona silty clay is studied focusing on the effects of the initial state and the microstructure set on compaction. The Proctor plane is used to map these states, which cover a wide range of degrees of saturation. Stiffness data at different confining stresses are determined using a resonant column apparatus. The microstructural characterization of the material is carried out by mercury intrusion and extrusion porosimetry tests and water retention curves. Results indicate an increase in the small-strain shear modulus with the reduction of degree of saturation for samples prepared at degrees of saturation greater than 0.3. The results are interpreted considering the evolution of the microstructure, and a constitutive stress incorporating degree of saturation and microstructural features. Based on these aspects, a microstructural framework is proposed to interpret small-strain stiffness. The characterization of the microstructural void ratio is critical in order to obtain appropriate results.