  The presence of primordial magnetic fields increases the minimum halo mass in
which star formation is possible at high redshifts. Estimates of the dynamical
mass of ultra-faint dwarf galaxies (UFDs) within their half-light radius
constrain their virialized halo mass before their infall into the Milky Way.
The inferred halo mass and formation redshift of the UFDs place upper bounds on
the primordial comoving magnetic field, $B_0$. We derive an upper limit of
$0.50\pm 0.086$ ($0.31\pm 0.04$) nG on $B_0~$ assuming the average formation
redshift of the UFD host halos is $z_{\rm form}=$ 10 (20), respectively.
