  We present the first JWST spectral energy distribution of a Y dwarf. This
spectral energy distribution of the Y0 dwarf WISE J035934.06$-$540154.6
consists of low-resolution ($\lambda$/$\Delta\lambda$ $\sim$ 100) spectroscopy
from 1$-$12 $\mu$m and three photometric points at 15, 18, and 21 $\mu$m. The
spectrum exhibits numerous fundamental, overtone, and combination
rotational-vibrational bands of H$_2$O, CH$_4$, CO, CO$_2$, and NH$_3$,
including the previously unidentified $\nu_3$ band of NH$_3$ at 3 $\mu$m. Using
a Rayleigh-Jeans tail to account for the flux emerging at wavelengths greater
than 21 $\mu$m, we measure a bolometric luminosity of
$1.523\pm0.090\times10^{20}$ W. We determine a semi-empirical effective
temperature estimate of $467^{+16}_{-18}$ K using the bolometric luminosity and
evolutionary models to estimate a radius. Finally, we compare the spectrum and
photometry to a grid of atmospheric models and find reasonably good agreement
with a model having $T_{\mathrm{eff}}$=450 K, log $g$=3.25 [cm s$^{-2}$],
[M/H]=$-0.3$. However, the low surface gravity implies an extremely low mass of
1 $M_{\rm{Jup}}$ and a very young age of 20 Myr, the latter of which is
inconsistent with simulations of volume-limited samples of cool brown dwarfs.
