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Centaurs are minor solar system bodies with orbits transitioning between those of trans-Neptunian scattered disk objects and Jupiter-family comets (JFCs). 39P/Oterma (39P) is a frequently active centaur that has recently held both centaur and JFC classifications and was observed with the JWST NIRSpec instrument on 2022 July 27 UTC while it was 5.82 au from the Sun. For the first time, CO₂gas emission was detected in a centaur, with a production rate of$Q_{{\mathrm{CO}}_2}$= (5.96 ± 0.80) × 10²³molecules s⁻¹. This is the lowest detection of CO₂of any centaur or comet. CO and H₂O were not detected down to constraining upper limits. Derived mixing ratios ofQ_CO/Q${}_{{\mathrm{CO}}_2}$≤ 2.03 and$Q_{{\mathrm{CO}}_2}$/Q${}_{{\mathrm{H}}_2\mathrm{O}}$≥ 0.60 are consistent with CO₂and/or CO outgassing playing large roles in driving the activity, but not water, and show a significant difference between the coma abundances of 29P/Schwassmann–Wachmann 1, another centaur at a similar heliocentric distance, which may be explained by thermal processing of 39P’s surface during its previous JFC orbit. To help contextualize the JWST data we also acquired visible CCD imaging data on two dates in 2022 July (Gemini-North) and September (Lowell Discovery Telescope). Image analysis and photometry based on these data are consistent with a point-source detection and an estimated effective nucleus radius of 39P in the range ofR_nuc= 2.21–2.49 km.