A modular armor system configured to be readily attached and detached from a frame surrounding a window in a vehicle or other structure, such as a building. The modular armor system may be configured to provide any desired ballistics protection rating. In one embodiment, the modular armor system includes a ballistics-grade armor panel having an outer strike face and an inner surface opposite the outer strike face. The modular armor system also includes a fastener coupled to the ballistics-grade armor panel. The fastener is configured to detachably couple the ballistics-grade armor panel to the frame surrounding the window in the vehicle or other structure.