  DH Tau b is a young planetary-mass companion orbiting at a projected
separation of 320 AU from its $\sim$2 Myr old host star DH Tau. With an
estimated mass of $8-22$ $M_{\rm{Jup}}$ this object straddles the
deuterium-burning limit, and might have formed via core or pebble accretion,
disk instability, or molecular cloud fragmentation. To shed light on the
formation history of DH Tau b, we obtain the first measurement of rotational
line broadening for this object using high-resolution (R $\sim$25,000)
near-infrared spectroscopy from Keck/NIRSPEC. We measure a projected rotational
velocity ($v$sin$i$) of $9.6\pm0.7$ km/s, corresponding to a rotation rate that
is between 9-15% of DH Tau b's predicted break-up speed. This low rotation rate
is in good agreement with scenarios in which magnetic coupling between the
companion and its circumplanetary disk during the late stages of accretion
reduces angular momentum and regulates spin. We compare the rotation rate of DH
Tau b to published values for other planetary-mass objects with masses between
$0.3-20$ $M_{\rm{Jup}}$ and find no evidence of a correlation between mass and
rotation rate in this mass regime. Finally, we search for evidence of
individual molecules in DH Tau b's spectrum and find that it is dominated by CO
and H$_2$O, with no evidence for the presence of CH$_4$. This agrees with
expectations given DH Tau b's relatively high effective temperature ($\sim$2300
K).
