Recently, complementary metal–oxide–semiconductor (CMOS) sensors have progressed to a point where they may offer improved performance in imaging x-ray detection compared to the charge-coupled devices often used in x-ray satellites. We demonstrate x-ray detection in the soft x-ray band (250 to 1700 eV) by a commercially available back-illuminated Sony IMX290LLR CMOS sensor using the Advanced Photon Source at Argonne National Laboratory. While operating the device at room temperature, we measure energy resolutions (full width at half maximum) of 48 eV at 250 eV and of 83 eV at 1700 eV, which are comparable to the performance of the “Chandra” ACIS and the “Suzaku” XIS. Furthermore, we demonstrate that the IMX290LLR can withstand radiation up to 17.1 krad, making it suitable for use on spacecraft in low Earth orbit.