<p>Small-strain behavior of soils, for example, shear modulus, is a key parameter in marine geological engineering, as it governs the deformation response of offshore foundations. Fines can alter the mechanical performance of soils, yet the stiffness in elastic strain range of biotreated sands with fines is seldom reported. In the current study, the small-strain shear modulus of biotreated carbonate sands with non-plastic fines was explored through isotropic consolidation tests. Experimental results show that fines content influences both the small-strain modulus and stiffness anisotropy. Biotreatment enhances the wave velocity of sands, particularly in silty sands, yet the modulus still declines as the fines content increases. The modulus shows two distinct stress-dependent regimes, and stress history has different impacts on the modulus of untreated and biotreated sands. A modified modulus prediction model based on the unified equivalent intergranular void ratio, incorporating the contributions of fines and CaCO₃ precipitation, is proposed. The paper innovatively presents the stiffness and its prediction model of biotreated carbonate silty sands in small-strain field, which contributes the estimation of deformation of marine geological infrastructure.</p>