  Measuring interstellar magnetic fields is extremely important for
understanding their role in different evolutionary stages of interstellar
clouds and of star formation. However, detecting the weak field is
observationally challenging. We present measurements of the Zeeman effect in
the 1665 and 1667~MHz (18~cm) lines of the hydroxyl radical (OH) lines toward
the dense photodissociation region (PDR) associated with the compact H{\sc ii}
region DR~21~(Main). From the OH 18~cm absorption, observed with the Karl G.
Jansky Very Large Array, we find that the line of sight magnetic field in this
region is $\sim 0.13$~mG. The same transitions in maser emission toward the
neighboring DR~21(OH) and W~75S-FR1 regions also exhibit the Zeeman splitting.
Along with the OH data, we use [C{\sc ii}] 158 $\mu$m line and hydrogen radio
recombination line data to constrain the physical conditions and the kinematics
of the region. We find the OH column density to be $\sim
3.6\times10^{16}(T_{\rm ex}/25~{\rm K})~{\rm cm}^{-2}$, and that the 1665 and
1667 MHz absorption lines are originating from the gas where OH and C$^+$ are
co-existing in the PDR. Under reasonable assumptions, we find the measured
magnetic field strength for the PDR to be lower than the value expected from
the commonly discussed density--magnetic field relation while the field
strength values estimated from the maser emission are roughly consistent with
the same. Finally, we compare the magnetic field energy density with the
overall energetics of DR~21's PDR and find that, in its current evolutionary
stage, the magnetic field is not dynamically important.
