  We study the effects of strong magnetic fields on the properties of hyperonic
matter. We employ the relativistic mean field theory, which is known to provide
excellent descriptions of nuclear matter and finite nuclei. The two additional
hidden-strangeness mesons, $\sigma^{\ast}$ and $\phi$, are taken into account,
and some reasonable hyperon potentials are used to constrain the meson-hyperon
couplings, which reflect the recent developments in hypernuclear physics. It is
found that the effects of strong magnetic fields become significant only for
magnetic field strength $B>5 \times 10^{18}$ G. The threshold densities of
hyperons can be significantly altered by strong magnetic fields. The presence
of hyperons makes the equation of state (EOS) softer than that in the case
without hyperons, and the softening of the EOS becomes less pronounced with
increasing magnetic field strength.
