Atmospheric metal enrichment (that is, elements heavier than helium, also called `metallicity') is a key diagnostic of the formation of giant planets1-3. The giant planets of the Solar System show an inverse relationship between mass and both their bulk metallicities and atmospheric metallicities. Extrasolar giant planets also display an inverse relationship between mass and bulk metallicity(4). However, there is significant scatter in the relationship and it is not known how atmospheric metallicity correlates with either planet mass or bulk metallicity. Here we show that the Saturn-mass exoplanet HD 149026b (refs. 5-9) has an atmospheric metallicity 59-276 times solar (at 1 sigma), which is greater than Saturn's atmospheric metallicity of roughly 7.5 times solar(10) at more than 4 sigma confidence. This result is based on modelling CO2 and H2O absorption features in the thermal emission spectrum of the planet measured by the James Webb Space Telescope. HD 149026b is the most metal-rich giant planet known, with an estimated bulk heavy element abundance of 66 +/- 2% by mass(11,12). We find that the atmospheric metallicities of both HD 149026b and the Solar System giant planets are more correlated with bulk metallicity than planet mass.