  We present the direct-imaging discovery of a substellar companion in orbit
around a Sun-like star member of the Hyades open cluster. So far, no other
substellar companions have been unambiguously confirmed via direct imaging
around main-sequence stars in Hyades. The star HIP 21152 is an accelerating
star as identified by the astrometry from the Gaia and Hipparcos satellites. We
have detected the companion, HIP 21152 B, in multi-epoch using the
high-contrast imaging from SCExAO/CHARIS and Keck/NIRC2. We have also obtained
the stellar radial-velocity data from the Okayama 188cm telescope. The CHARIS
spectroscopy reveals that HIP 21152 B's spectrum is consistent with the L/T
transition, best fit by an early T dwarf. Our orbit modeling determines the
semi-major axis and the dynamical mass of HIP 21152 B to be
17.5$^{+7.2}_{-3.8}$ au and 27.8$^{+8.4}_{-5.4}$ $M_{\rm{Jup}}$, respectively.
The mass ratio of HIP 21152 B relative to its host is $\approx$2\%, near the
planet/brown dwarf boundary suggested from recent surveys. Mass estimates
inferred from luminosity evolution models are slightly higher (33--42
$M_{\rm{Jup}}$). With a dynamical mass and a well-constrained age due to the
system's Hyades membership, HIP 21152 B will become a critical benchmark in
understanding the formation, evolution, and atmosphere of a substellar object
as a function of mass and age. Our discovery is yet another key
proof-of-concept for using precision astrometry to select direct imaging
targets.
