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 _ { form } = 10 ( 20 ) , respectively .