© 2019 SPIE.We present an optomechanical impact sensor, designed by the utilization of a 2D rod-type photonic crystal (PhC) cavity. The PhC cavity is sandwiched by perfect electrical conductor (PEC) boundaries with an air slot between the top of the PhC rods and the bottom of the top PEC layer. Strong light localization in the air slot region makes the PhC cavity characteristics highly sensitive to the air slot width, leading to optomechanical applications such as impact sensing. A suspended mechanical gold membrane, as a replacement of PEC layers for practical realizations, is designed to sense impact acceleration. In the presence of an impact, the mechanical structure deflects resulting in a change in the air slot height, which in turn, tunes the resonant wavelength of the PhC cavity. Calculations show that 16.6 μs response time, much faster than the commercially available ones (around 200 ms), is possible.