  While the population of confirmed exoplanets continues to grow, the sample of
confirmed transiting planets around evolved stars is still limited. We present
the discovery and confirmation of a hot Jupiter orbiting TOI-2184 (TIC
176956893), a massive evolved subgiant ($M_\star= 1.53 \pm 0.12 M_\odot$,
$R_\star= 2.90 \pm 0.14 R_\odot$) in the $\textit{TESS}$ Southern Continuous
Viewing Zone. The planet was flagged as a false positive by the $\textit{TESS}$
Quick-Look Pipeline due to periodic systematics introducing a spurious depth
difference between even and odd transits. Using a new pipeline to remove
background scattered light in $\textit{TESS}$ Full Frame Image (FFI) data, we
combine space-based $\textit{TESS}$ photometry, ground-based photometry, and
ground-based radial velocity measurements to report a planet radius of $R_p=
1.017 \pm 0.051 R_J$ and mass of $M_p= 0.65 \pm 0.16 M_J$. For a planet so
close to its star, the mass and radius of TOI-2184b are unusually well matched
to those of Jupiter. We find that the radius of TOI-2184b is smaller than
theoretically predicted based on its mass and incident flux, providing a
valuable new constraint on the timescale of post-main-sequence planet
inflation. The discovery of TOI-2184b demonstrates the feasibility of detecting
planets around faint ($\textit{TESS}$ magnitude $>12$) post-main sequence stars
and suggests that many more similar systems are waiting to be detected in the
$\textit{TESS}$ FFIs, whose confirmation may elucidate the final stages of
planetary system evolution.
