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Isotope abundance ratios have an important role in astronomy and planetary sciences, providing insights into the origin and evolution of the Solar System, interstellar chemistry and stellar nucleosynthesis 1,2 . In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (around 89) in the Solar System 1,3 , but do vary on galactic scales with a 12 C/ 13 C isotopologue ratio of around 68 in the current local interstellar medium 4-6 . In molecular clouds and protoplanetary disks, 12 CO/ 13 CO ratios can be altered by ice and gas partitioning 7 , low-temperature isotopic ion-exchange reactions 8 and isotope-selective photodissociation 9 . Here we report observations of 13 CO in the atmosphere of the young, accreting super-Jupiter TYC 8998-760-1 b, at a statistical significance of more than six sigma. Marginalizing over the planet's atmospheric temperature structure, chemical composition and spectral calibration uncertainties suggests a 12 CO/ 13 CO ratio of [Formula: see text](90% confidence), a substantial enrichment in 13 C with respect to the terrestrial standard and the local interstellar value. As the current location of TYC 8998-760-1 b at greater than or equal to 160 astronomical units is far beyond the CO snowline, we postulate that it accreted a substantial fraction of its carbon from ices enriched in 13 C through fractionation., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)