Fig. 3: Interpretation of WASP-39b’s transmission spectrum.Carbon dioxide (CO$_2$) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO$_2$ is an indicator of the metal enrichment (that is, elements heavier than helium, also called `metallicity’)$^1-3$, and thus the formation processes of the primary atmospheres of hot gas giants$^4-6$. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets$^7-9$. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO$_2$, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification$^10-12$. Here we present the detection of CO$_2$ in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme$^13,14$. The data used in this study span 3.0-5.5 micrometres in wavelength and show a prominent CO$_2$ absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten- times solar metallicity models that assume radiative-convective- thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO$_2$, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models.