Hexagonal Ag$_3$YCl$_6$ was found to transform into a new phase by applying pressure above ~1.5 GPa via a diamond anvil cell. In-situ XRD analysis revealed that the phase is monoclinic Ag$_3$YCl$_6$. The bulk modulus of the hexagonal and monoclinic phases was 37 GPa and 49 GPa, respectively. Computational investigations also supported this phase transition and bulk modulus. After releasing pressure from 2 GPa, the diffraction peaks of hexagonal Ag$_3$YCl$_6$ were detected, indicating the reversibility of this phase transformation.