Pure body-center-cubic structured metastable polycrystalline Li15Si4 was synthesized in a Li-ion battery by electrochemically lithiating a thin Si wafer. Its stability and compressional behavior were investigated using in-situ high-pressure synchrotron X-ray diffraction (XRD) at room temperature. No phase transition was observed in polycrystalline Li15Si4 up to 5.8 GPa. A bulk modulus of 28.4(6) GPa was obtained for Li15Si4 by fitting the XRD data to a third order Birch–Murnaghan equation of state. We found that the bulk modulus of Li15Si4 follows a linear interpolation relationship between the bulk moduli for pure Li and Si. These results provide valuable experimental data to validate the theoretical calculation and are also important inputs for modeling the Li–Si system.