CIS221-X is a prototype complementary metal-oxide-semiconductor (CMOS) image sensor, optimized for soft x-ray astronomy and developed for the proposed ESA Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission. The sensor features 40 μm pitch square pixels built on a 35 μm thick, high-resistivity epitaxial silicon that is fully depleted by reverse substrate bias. Backside illumination processing has been used to achieve high x-ray quantum efficiency, and an optical light-blocking filter has been applied to mitigate the influence of stray light. A comprehensive electro-optical characterization of CIS221-X has been completed. The median readout noise is 3.3 e ⁻ RMS with 90% of pixels reporting a value <3.6 e ⁻ RMS. At −40 ° C, the dark current is 12.4 ± 0.06 e ⁻ / pixel / s. The pixel photo-response is linear to within 1% for 0.3 to 5 keV photons (82 to 1370 e ⁻ ) with <0.1 % image lag. Following per-pixel gain correction, an energy resolution of 130.2 ± 0.4 eV has been measured at 5898 eV. In the 0.3 to 1.8 keV energy range, CIS221-X achieves >80 % quantum efficiency. With the exception of dark current, these results either meet or outperform the requirements for the THESEUS mission, strongly supporting the consideration of CMOS technology for soft x-ray astronomy.