  We report the discovery of ZTF J2020+5033, a high-mass brown dwarf (BD)
transiting a low-mass star with an orbital period of 1.90 hours. Phase-resolved
spectroscopy, optical and infrared light curves, and precise astrometry from
Gaia allow us to constrain the masses, radii, and temperatures of both
components with few-percent precision. We infer a BD mass of $M_{\rm BD} =
80.1\pm 1.6\,M_{\rm J}$, almost exactly at the stellar/substellar boundary, and
a moderately inflated radius, $R_{\rm BD} = 1.05\pm 0.02\,R_{\rm J}$. The
transiting object's temperature, $T_{\rm eff}\approx 1700\,\rm K$, is
well-constrained by the depth of the infrared secondary eclipse and strongly
suggests it is a BD. The system's high tangential velocity ($v_\perp = 98\,\rm
km\,s^{-1}$) and thick disk-like Galactic orbit imply the binary is old; its
close distance ($d\approx 140$ pc) suggests that BDs in short-period orbits are
relatively common. ZTF J2020+5033 is the shortest-period known transiting BD by
more than a factor of 7. Today, the entire binary would comfortably fit inside
the Sun. However, both components must have been considerably larger in youth,
implying that the orbit has shrunk by at least a factor of $\sim 5$ since
formation. The simplest explanation is that magnetic braking continues to
operate efficiently in at least some low-mass stars and BDs.
