We report the first measurement of the charm-mixing parameter yCP in D0 decays to the CP-odd final state KS0ω. The study uses the full Belle e+e- annihilation data sample of 976 fb-1 taken at or near the (4S) centre-of-mass energy. We find yCP=(0.96±0.91±0.62-0.00+0.17)%, where the first uncertainty is statistical, the second is systematic due to event selection and background, and the last is due to possible presence of CP-even decays in the data sample. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.