  In the framework of the Nambu--Jona-Lasino (NJL) model, we study the effect
of an intense external uniform magnetic field on neutral and charged pion
masses and decay form factors. In particular, the treatment of charged pions is
carried out on the basis of the Ritus eigenfunction approach to magnetized
relativistic systems. Our analysis shows that in the presence of the magnetic
field three and four nonvanishing pion-to-vacuum hadronic form factors can be
obtained for the case of the neutral and charged pions, respectively. As
expected, it is seen that for nonzero magnetic field the $\pi^0$ meson can
still be treated as a pseudo Nambu-Goldstone boson, and consequently the
corresponding form factors are shown to satisfy various chiral relations. For
definite parametrizations of the model, numerical results for $\pi^0$ and
$\pi^\pm$ masses and decay constants are obtained and compared with previous
calculations given in the literature.
