  Model-independent masses of substellar companions are critical tools to
validate models of planet and brown dwarf cooling, test their input physics,
and determine the formation and evolution of these objects. In this work, we
measure the dynamical mass and orbit of the young substellar companion HD 984
B. We obtained new high-contrast imaging of the HD 984 system with Keck/NIRC2
which expands the baseline of relative astrometry from 3 to 8 years. We also
present new radial velocities of the host star with the Habitable-Zone Planet
Finder spectrograph at the Hobby-Eberly Telescope. Furthermore, HD 984 exhibits
a significant proper motion difference between Hipparcos and Gaia EDR3. Our
joint orbit fit of the relative astrometry, proper motions, and radial
velocities yields a dynamical mass of $61 \pm 4$ $\mathrm{M_{Jup}}$ for HD 984
B, placing the companion firmly in the brown dwarf regime. The new fit also
reveals a higher eccentricity for the companion ($e = 0.76 \pm 0.05$) compared
to previous orbit fits. Given the broad age constraint for HD 984, this mass is
consistent with predictions from evolutionary models. HD 984 B's dynamical mass
places it among a small but growing list of giant planet and brown dwarf
companions with direct mass measurements.
