Abstract The atmosphere of Mars is mainly composed by carbon dioxide (CO₂). It has been predicted that photodissociation of CO₂ depletes ¹³C in carbon monoxide (CO). We present the carbon ¹³C/¹²C isotopic ratio in CO at 30–50 km altitude from the analysis of the solar occultation measurements taken by the instrument Nadir and Occultation for Mars Discovery on board the ExoMars Trace Gas Orbiter (ExoMars-TGO). We retrieve ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O volume mixing ratios from the spectra taken at 4112–4213 cm⁻¹, where multiple CO isotope lines with similar intensities are available. The intensities of the ¹²C¹⁶O lines in this spectral range are particularly sensitive to temperature, thus we derive the atmospheric temperature by retrieving CO₂ density with simultaneously measured spectra at 2966–2990 cm⁻¹. The mean δ ¹³C value obtained from the ¹³C¹⁶O/¹²C¹⁶O ratios is −263‰, and the standard deviation and standard error of the mean are 132‰ and 4‰, respectively. The relatively large standard deviation is due to the strong temperature dependences in the ¹²C¹⁶O lines. We also examine the ¹³C¹⁶O/¹²C¹⁸O ratio, whose lines are less sensitive to temperature. The mean δ value obtained with ¹²C¹⁸O instead of ¹²C¹⁶O is −82‰ with smaller standard deviation, 60‰. These results suggest that CO is depleted in ¹³C when compared to CO₂ in the Martian atmosphere as measured by the Curiosity rover. This depletion of ¹³C in CO is consistent with the CO₂ photolysis-induced fractionation, which might support a CO-based photochemical origin of organics in Martian sediments.