  We present the dayside thermal emission spectrum of WASP-77Ab from 2.8 -- 5.2
$\mu$m as observed with the NIRSpec instrument on the James Webb Space
Telescope (JWST). WASP-77Ab was previously found to have a sub-solar
metallicity and a solar carbon-to-oxygen (C/O) ratio from H$_2$O and CO
absorption lines detected using high-resolution spectroscopy. By performing
atmospheric retrievals on the JWST spectrum assuming chemical equilibrium, we
find a sub-solar metallicity [M/H]=$-0.91^{+0.24}_{-0.16}$ and C/O ratio
$0.36^{+0.10}_{-0.09}$. We identify H$_2$O and CO and constrain their
abundances, and we find no CO$_2$ in the spectrum. The JWST and high-resolution
spectroscopy results agree within $\sim1\sigma$ for the metallicity and within
1.8$\sigma$ for the C/O ratio. However, our results fit less well in the
picture painted by the shorter wavelength spectrum measured by HST WFC3.
Comparing the JWST thermal emission spectra of WASP-77Ab and HD 149026b shows
that both hot Jupiters have nearly identical brightness temperatures in the
near-infrared, but distinctly different atmospheric compositions. Our results
reaffirm high-resolution spectroscopy as a powerful and reliable method to
measure molecular abundances. Our results also highlight the incredible
diversity of hot Jupiter atmospheric compositions.
