As biocementation is beginning to be explored to improve the engineering properties of fine-grained soils such as clays, its performance under unsaturated conditions has become a key concern. In these geomaterials, the coupled interaction between the liquid and gas phases and their interaction with the solid skeleton plays a key role in the overall hydromechanical response. Accordingly, establishing a fundamental understanding of the unsaturated mechanical behavior of biocemented clays requires examining how calcium carbonate precipitation influences water retention and volumetric behavior. This study investigates these effects using two clayey soils biocemented at calcite contents of 5% and 10% using a novel mixing technique. Experimental results show that biocementation significantly reduces volumetric shrinkage under free-drying conditions, with volumetric strain reductions of up to 25%. In addition, treated soils exhibit a pronounced increase in air-entry value (AEV), reaching approximately a 70% increase at the 10% calcite level. Crucially, this increase in AEV is found to be independent of clay type, suggesting a generalizable trend for these geomaterials. By providing a detailed characterization of the water retention curves and volumetric response, this research offers a framework for understanding the unsaturated behavior of biocemented fine-grained soils. Ultimately, these results support the development of more reliable biocementation approaches for clays subjected to moisture variations, facilitating the broader adoption of this technique. • Developed a novel mixing protocol for biocementation, using ex-situ urea hydrolysis, that ensures uniform calcite distribution in clays and reduces environmental impact. • Biocementation reduced volumetric shrinkage by up to 25% in low-plasticity clay (LPC) and ~12% in high-plasticity clay (HPC) under free drying. • Air entry value (AEV) increased by ~40% at 5% calcite content and ~70% at 10% calcite content for both clay types. • Results suggest biocementation affects water retention and volumetric behavior through distinct microstructural changes to the clay fabric