The atmospheric transmission spectrum of the sub-Neptune exoplanet GJ 3470 b was observed using the Near-Infrared Camera (NIRCam) on the James Webb Space Telescope (JWST). Combined with two archival Hubble Space Telescope (HST) and fifteen archival Spitzer observations, the study detected water, methane, sulfur dioxide, and carbon dioxide in the atmosphere of GJ 3470 b, each with a significance of >3σ. GJ 3470 b is the lowest mass and coldest exoplanet known to show a substantial sulfur dioxide feature in its spectrum, with a mass of 11.2 M⊕ and an equilibrium temperature of 600 K. This indicates that disequilibrium photochemistry drives sulfur dioxide production in exoplanet atmospheres over a wider range of masses and temperatures than previously reported or expected. The measured abundances of water, carbon dioxide, and sulfur dioxide suggest an atmospheric metallicity of approximately 100× Solar. Further evidence for disequilibrium chemistry is seen in the inferred methane abundance, which is significantly lower than expected from equilibrium models consistent with the measured water and carbon dioxide abundances. The study used a combination of new JWST/NIRCam data and archival HST/WFC3 and Spitzer transit observations to analyze the transmission spectrum of GJ 3470 b. The data were re-extracted and re-fitted to minimize possible offsets between different observations. The stellar spectral energy distribution (SED) was fitted using the Gaia parallax to determine the stellar radius. The results from the joint fit of broadband photometry were consistent with previous measurements, with slight adjustments in the planetary orbit and transit depth due to new NIRCam data. The spectroscopic transit depths of GJ 3470 b were measured using the JWST/NIRCam observations and archival HST/WFC3 data. The results showed a mild shallower depth in the WFC3 spectrum compared to previous reports, due to more refined measurements of a/R* and cos(i). The stellar SED modeling allowed the measurement of the star GJ 3470's radius as 0.50 ± 0.01 R☉. The planetary radius of GJ 3470 b was estimated as 4.22 ± 0.09 R⊕. The stellar mass was constrained to be 0.44 ± 0.04 M☉, and the planetary mass was estimated as 11.1 ± 0.9 M⊕. The atmospheric retrievals used these absolute stellar and planetary radii and masses. The stellar variability monitoring showed sinusoidal brightness modulations with an amplitude of 2545 ± 130 ppm at a period of 21.6239 ± 0.0014 days. This variability amplitude is roughly halfThe atmospheric transmission spectrum of the sub-Neptune exoplanet GJ 3470 b was observed using the Near-Infrared Camera (NIRCam) on the James Webb Space Telescope (JWST). Combined with two archival Hubble Space Telescope (HST) and fifteen archival Spitzer observations, the study detected water, methane, sulfur dioxide, and carbon dioxide in the atmosphere of GJ 3470 b, each with a significance of >3σ. GJ 3470 b is the lowest mass and coldest exoplanet known to show a substantial sulfur dioxide feature in its spectrum, with a mass of 11.2 M⊕ and an equilibrium temperature of 600 K. This indicates that disequilibrium photochemistry drives sulfur dioxide production in exoplanet atmospheres over a wider range of masses and temperatures than previously reported or expected. The measured abundances of water, carbon dioxide, and sulfur dioxide suggest an atmospheric metallicity of approximately 100× Solar. Further evidence for disequilibrium chemistry is seen in the inferred methane abundance, which is significantly lower than expected from equilibrium models consistent with the measured water and carbon dioxide abundances. The study used a combination of new JWST/NIRCam data and archival HST/WFC3 and Spitzer transit observations to analyze the transmission spectrum of GJ 3470 b. The data were re-extracted and re-fitted to minimize possible offsets between different observations. The stellar spectral energy distribution (SED) was fitted using the Gaia parallax to determine the stellar radius. The results from the joint fit of broadband photometry were consistent with previous measurements, with slight adjustments in the planetary orbit and transit depth due to new NIRCam data. The spectroscopic transit depths of GJ 3470 b were measured using the JWST/NIRCam observations and archival HST/WFC3 data. The results showed a mild shallower depth in the WFC3 spectrum compared to previous reports, due to more refined measurements of a/R* and cos(i). The stellar SED modeling allowed the measurement of the star GJ 3470's radius as 0.50 ± 0.01 R☉. The planetary radius of GJ 3470 b was estimated as 4.22 ± 0.09 R⊕. The stellar mass was constrained to be 0.44 ± 0.04 M☉, and the planetary mass was estimated as 11.1 ± 0.9 M⊕. The atmospheric retrievals used these absolute stellar and planetary radii and masses. The stellar variability monitoring showed sinusoidal brightness modulations with an amplitude of 2545 ± 130 ppm at a period of 21.6239 ± 0.0014 days. This variability amplitude is roughly half