We present TOI-2155 b, a massive transiting companion, discovered using data from NASA’s Transiting Exoplanet Survey Satellite mission and confirmed with ground-based radial velocity measurements from the Tillinghast Reflector Echelle Spectrograph. We also analyze ground-based follow-up photometric data from the Wendelstein Observatory, Las Cumbres Observatory Global Telescope, and Wild Boar Remote Observatory. TOI-2155 b is a short-period companion with P = 3.7246950 ± 0.0000014 days. The radius and mass of TOI-2155 b are found to be

R_{b} = 0.97 2_{- 0.008}^{+ 0.009} R_{J}

and

M_{b} = 80 . 6_{- 1.1}^{+ 1.0} M_{J}

, respectively, corresponding to a density of

ρ_{b} = 10 9_{- 3.3}^{+ 3.1}

g cm ⁻³ . The F-type subgiant host star has an effective temperature of T _eff = 6085 ± 78 K, a radius

R_{⋆} = 1.70 5_{- 0.064}^{+ 0.066}

R _⊙ , and a mass M _⋆ = 1.33 ± 0.008 M _⊙ . With a mass close to the hydrogen-burning minimum mass, TOI-2155 b lies at the boundary between brown dwarfs and low-mass stars. Its measured mass, radius, and density place it in a transitional region, where distinguishing between a massive brown dwarf and a very low-mass star is not straightforward. TOI-2155 b therefore provides a valuable benchmark for testing evolutionary models of stellar and substellar structure near the hydrogen-burning limit.

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