<p>In coastal regions, calcareous sands, which differ mechanically from siliceous soils, can be affected by hydrocarbon contamination, potentially altering their dynamic behavior and reducing the seismic stability of infrastructure. Despite its importance, the dynamic response of calcareous sands under hydrocarbon exposure has received limited attention. This research investigates the strain-dependent dynamic properties of Chabahar calcareous sand contaminated with varying amounts of light crude oil and gas oil. Resonant column and cyclic triaxial experiments were conducted at three confining pressures (75, 150, and 300&#xa0;kPa) and contamination levels of 0, 4, 8, and 12%, focusing on variations in shear modulus and damping ratio. The results indicate divergent effects from the two contaminants: crude oil appears to increase shear modulus and reduce damping, whereas gas oil decreases shear modulus and raises damping. These contrasting behaviors are linked to microscale interactions—crude oil may promote temporary cohesion, thin viscous film formation, and meniscus-induced bonding at water–hydrocarbon–particle interfaces, enhancing interparticle cohesion and mechanical integrity, while gas oil primarily acts as a lubricant, reducing intergranular friction. The mechanisms were investigated using Fourier Transform Infrared Spectroscopy and optical microscopy. Based on the experimental results, a strain-dependent modified hyperbolic constitutive model was proposed, integrating both contamination level and applied confining pressure. The model exhibited superior accuracy (R² &gt; 0.999; RMSE &lt; 0.0013), outperforming traditional predictive frameworks. This study pioneers the integration of microscale observations into a macroscopic constitutive model for hydrocarbon-contaminated calcareous sands, offering a valuable tool for geotechnical assessment and design in polluted environments.</p>