Commercially manufactured complementary metal–oxide–semiconductor (CMOS) sensors have demonstrated competitive x-ray spectral imaging performance to the charge-coupled devices flown on the Suzaku and Chandra missions without the cooling demands required of these sensors. This performance, in combination with their reduced costs, warrants regarding CMOS sensors as promising candidates for low-Earth orbit (LEO) x-ray small satellites. We investigate the radiation tolerance of these devices to the anticipated total ionizing dose (TID) radiation expected in LEO. We expose a backside-illuminated Sony IMX290LLR CMOS sensor to up to 12 krad of TID from Cs137 gamma-ray radiation. We find an increase in the abundance of noisy pixels with increasing dosage, but no discernible increase in the average dark signal or RMS noise. Measurements of the x-ray spectrum from a Fe55 source indicate no change in spectral resolution and only minor gain degradation with TID.