Modulus reduction and damping (MRD) curves are essential in estimating the response of soils to earthquake ground shaking in seismic site response analysis. Microbial-induced carbonate precipitation (MICP) is a biological grouting method, with demonstrated effectiveness in shear strength improvement and potential for liquefaction mitigation. However, the effect of MICP on seismic site response has rarely been studied, and the MRD formulation for MICP-treated soil (MRDMICP) applicable to numerical site response analysis has yet to be developed. To overcome this hurdle, (1) we provide a best-fitting MRD formulation of MICP-treated sand for varying cementation levels at a constant confining pressure based on laboratory dynamic tests, and (2) we used the proposed formulation in simulating a one-dimensional nonlinear site response analysis for a hypothetical MICP-treated site. We assumed a 35 m-deep soil column composed of saturated, homogenous, and poorly graded sand that will be locally treated with MICP a few meters above the assumed bedrock for the site response analysis. Outcrop ground motions with weak and strong shaking intensities were used as input at the bedrock level. Nonlinear site response analysis showed a reduced pore water pressure (PWP) ratio at the MICP-treated layers. However, the PWP ratio and ground motions at the surface layer (that remained untreated) experienced a rebound and amplification, respectively. Optimum cementation levels that minimize the amplitude of surface ground motions and reduce the PWP ratio at the soil layers were different when considering different levels of ground shaking. Hence, it is important to further elucidate the relationship between levels of cementation, hazard consistent ground motions, and potential site effects.