  We present the discovery and characterization of HIP 33609 b, a transiting
warm brown dwarf orbiting a late B star, discovered by NASA's Transiting
Exoplanet Survey Satellite TESS as TOI-588 b. HIP 33609 b is a large (R$_{b}$ =
1.580$_{-0.070}^{+0.074}$ R$_{J}$) brown dwarf on a highly eccentric (e =
0.560$_{-0.031}^{+0.029}$) orbit with a 39-day period. The host star is a
bright (V = 7.3 mag), T$_{eff}$ = 10,400$_{-660}^{+800}$ K star with a mass of
M$_{*}$ = 2.383$_{-0.095}^{+0.10}$ M$_{\odot}$ and radius of R$_{*}$ =
1.863$_{-0.082}^{+0.087}$ R$_{\odot}$, making it the hottest transiting brown
dwarf host star discovered to date. We obtained radial velocity measurements
from the CHIRON spectrograph confirming the companion's mass of M$_{b}$ =
68.0$_{-7.1}^{+7.4}$ M$_{J}$ as well as the host star's rotation rate
($vsini_{*} = 55.6 \pm 1.8$ km/s). We also present the discovery of a new
comoving group of stars, designated as MELANGE-6, and determine that HIP 33609
is a member. We use a combination of rotation periods and isochrone models fit
to the cluster members to estimate an age of 150 $\pm$ 25 Myr. With a measured
mass, radius, and age, HIP 33609 b becomes a benchmark for substellar
evolutionary models.
