  PSO J318.5338$-$22.8603 is an extremely-red planetary-mass object that has
been identified as a candidate member of the $\beta$ Pictoris moving group
based on its spatial position and tangential velocity. We present a high
resolution $K$-band spectrum of PSO J318.5338$-$22.8603. Using a
forward-modeling Markov Chain Monte Carlo approach, we report the first
measurement of the radial velocity and $v$ sin($i$) of PSO J318.5$-$22,
$-$6.0$^{+0.8}_{-1.1}$ km s$^{-1}$ and 17.5$^{+2.3}_{-2.8}$ km s$^{-1}$,
respectively. We calculate the space velocity and position of PSO J318.5$-$22
and confirm that it is a member of the $\beta$ Pictoris moving group. Adopting
an age of 23$\pm$3 Myr for PSO J318.5$-$22, we determine a mass of $8.3\pm0.5$
$M_{\rm{Jup}}$ and effective temperature of $1127^{+24}_{-26}$ K using
evolutionary models. PSO J318.5338$-$22.8603 is intermediate in mass and
temperature to the directly-imaged planets $\beta$ Pictoris b and 51 Eridani b,
making it an important benchmark object in the sequence of planetary-mass
members of the $\beta$ Pictoris moving group. Combining our $v$ sin($i$)
measurement with recent photometric variability data, we constrain the
inclination of PSO J318.5$-$22 to $>29^{\circ}$ and its rotational period to
5-10.2 hours. The equatorial velocity of PSO J318.5$-$22 indicates that its
rotation is consistent with an extrapolation of the velocity-mass relationship
for solar system planets.
