Indoor humidity exerts a significant influence on the durability of buildings and the health of occupants. A prominent approach to addressing this issue is the utilization of materials with hygroscopic properties for passive humidity regulation. Among these, all-coral concrete has emerged as a novel material particularly suited for hot and humid climates. However, research on its hygroscopic characteristics remains limited. Therefore, according to ISO standards, this paper undertook several tests to clarify the hygroscopic parameters of all-coral concrete (liquid water diffusivity, vapor permeability, isotherm sorption curve, and moisture buffer value). Based on these, the optimal moisture capacity was determined. Experimental results indicate that for the all-coral concrete, the liquid water diffusivity is 4.19 × 10–8 m/s, and the vapor permeability coefficient varies with relative humidity following the function: y = 7.04 × 10–11 + 2.97 × 10–10·x13.05. Furthermore, a notable hysteresis is observed between the absorption and desorption isotherms. The moisture buffer value was measured at 1.46 [g/(m2·%RH)], indicating the material's effective humidity-regulating performance and significant impact on environmental humidity. For moisture transfer calculations involving all-coral concrete, adopting the slope of the average isotherm curve enhances accuracy, yielding a discrepancy of only 2.4% compared to experimental results.