We present the results of a radiation test program for a 1-megapixel single-photon-counting and photon-number-resolving CMOS image sensor. The results include pre- and post-radiation values for dark current, voltage shift at the pixels’ output, read noise, quantum efficiency (QE), conversion gain, and photon counting ability. The Center for Detectors at the Rochester Institute of Technology exposed the sensor to a 50-krad(Si) dose of 60-MeV protons, equivalent to the dose absorbed over 10 11-year space missions at L2 with 1-cm aluminum shielding. The median dark current of the sensor increased from 0.00085 to 0.0085 e−/s/pix at 258 K and from 0.0075 to 0.075 e−/s/pix at 282 K. This is an increase of 2.0 fA/cm2/krad(Si) and 17.8 fA/cm2/krad(Si), respectively. Performance in other metrics remained constant: 0.34 e− median read noise, 85% peak QE at 490 nm, and photon number resolution. We report mostly total ionizing dose and displacement damage dose effects and compare the radiation tolerance of the device to the performance of state-of-the-art charge coupled devices and CMOS devices. The detector exhibits a comparable radiation tolerance to the expected tolerance of modern CMOS devices.