  We derive the non-thermal velocities (NTVs) in the transition region of an
active region using the \ion{Si}{4}~1393.78~{\AA} line observed by the
Interface Region Imaging Spectrograph (IRIS) and compare them with the
line-of-sight photospheric magnetic fields obtained by the Helioseismic and
Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The active
region consists of two strong field regions with opposite polarity, separated
by a weak field corridor, that widened as the active region evolved. The means
of the NTV distributions in strong-field regions (weak field corridors) range
between $\sim$18{--}20 (16{--}18)~km~s$^{-1}$, albeit the NTV maps show much
larger range. In addition, we identify a narrow lane in the middle of the
corridor with significantly reduced NTV. The NTVs do not show a strong
center-to-limb variation, albeit somewhat larger values near the disk center.
The NTVs are well correlated with redshifts as well as line intensities. The
results obtained here and those presented in our companion paper on Doppler
shifts suggest two populations of plasma in the active region emitting in
\ion{Si}{4}. The first population exists in the strong field regions and
extends partway into the weak field corridor between them. We attribute this
plasma to spicules heated to $\sim$0.1 MK (often called type II spicules). They
have a range of inclinations relative to vertical. The second population exists
in the center of the corridor, is relatively faint, and has smaller velocities,
likely horizontal. These results provide further insights into the heating of
the transition region.
