The use of X-ray radiation in radiotherapy is a common treatment for many cancers. Despite several scientific advances, determination of radiation delivered to the patient remains a challenge due to the inherent limitations of existing dosimeters including fabrication and operation. Here, we describe a colorimetric nanosensor that exhibits unique changes in color as a function of therapeutically relevant radiation dose (3-15 Gy). The nanosensor is formulated using a gold salt and maltose-binding protein as a templating agent, which upon exposure to ionizing radiation is converted to gold nanoparticles. The formation of gold nanoparticles from colorless precursor salts renders a change in color that can be observed visually. The dose-dependent multicolored response was quantified through a simple ultraviolet-visible spectrophotometer and the peak shift associated with the different colored dispersions was used as a quantitative indicator of therapeutically relevant radiation doses. The ease of fabrication, visual color changes upon exposure to ionizing radiation, and quantitative read-out demonstrates the potential of protein-facilitated biomineralization approaches to promote the development of next-generation detectors for ionizing radiation.