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.
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