  Atmospheric metal enrichment (i.e., elements heavier than helium, also called
"metallicity") is a key diagnostic of the formation of giant planets. The giant
planets of the solar system exhibit 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.
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 has an
atmospheric metallicity 59 - 276 times solar (at 1 $\sigma$), which is greater
than Saturn's atmospheric metallicity of ~7.5 times solar at >4 $\sigma$
confidence. This result is based on modeling CO$_2$ and H$_2$O absorption
features in the thermal emission spectrum of the planet measured by JWST. HD
149026b is the most metal-rich giant planet known, with an estimated bulk heavy
element abundance of 66 $\pm$ 2% by mass. 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.
