  We present the discovery of TOI-5205~b, a transiting Jovian planet orbiting a
solar metallicity M4V star, which was discovered using Transiting Exoplanet
Survey Satellite photometry and then confirmed using a combination of precise
radial velocities, ground-based photometry, spectra, and speckle imaging.
TOI-5205~b has one of the highest mass ratios for M dwarf planets with a mass
ratio of almost 0.3$\%$, as it orbits a host star that is just $0.392 \pm
0.015$ \solmass{}. Its planetary radius is $1.03 \pm 0.03~R_J$, while the mass
is $1.08 \pm 0.06~M_J$. Additionally, the large size of the planet orbiting a
small star results in a transit depth of $\sim 7\%$, making it one of the
deepest transits of a confirmed exoplanet orbiting a main-sequence star. The
large transit depth makes TOI-5205~b a compelling target to probe its
atmospheric properties, as a means of tracing the potential formation pathways.
While there have been radial-velocity-only discoveries of giant planets around
mid-M dwarfs, this is the first transiting Jupiter with a mass measurement
discovered around such a low-mass host star. The high mass of TOI-5205~b
stretches conventional theories of planet formation and disk scaling relations
that cannot easily recreate the conditions required to form such planets.
