  Recent observations of the rotation curve of M31 show a rise of the outer
part that can not be understood in terms of standard dark matter models or
perturbations of the galactic disc by M31's satellites. Here, we propose an
explanation of this dynamical feature based on the influence of the magnetic
field within the thin disc. We have considered standard mass models for the
luminous mass distribution, a NFW model to describe the dark halo, and we have
added up the contribution to the rotation curve of a magnetic field in the
disc, which is described by an axisymmetric pattern. Our conclusion is that a
significant improvement of the fit in the outer part is obtained when magnetic
effects are considered. The best-fit solution requires an amplitude of ~4
microG with a weak radial dependence between 10 and 38 kpc.
