In multiferroic TbMn2O5, the behavior of the terbium ions forms a crucial part of the magnetoelectric coupling. The result is a magnetically induced reversal of the electric polarization at 2 T. In this article we present the first direct measurement of the terbium magnetic structure under applied magnetic fields. Contrary to the current interpretation of the magnetic properties of TbMn2O5, we show that upon the electric polarization reversal the terbium sublattice adopts a canted antiferromagnetic structure with a large component of magnetic moment parallel to the a axis. Furthermore, we provide evidence for a coupling between the manganese 3d magnetic structure and the terbium 4f magnetism, which is of great significance in the elusive magnetoelectric mechanisms at play
