A large wavelength magnetic modulation, which has never been observed up to now, has been evidenced by neutron scattering experiments in (0001) terbium single crystal films. With decreasing temperature, this magnetic arrangement appears when the helical one vanishes and before the long range ferromagnetic order sets in. The large wavelength magnetic modulation is squared and it varies between 180 and 700 Å depending on the temperature and the film thickness. We assume that it is due to the stacking of ferromagnetic blocks whose net magnetization points along a basal plane direction and rotates by a given angle between two consecutive blocks. The occurrence of this phase seems to be independent of the strains induced by epitaxy but it would be related to the lattice clamping between epitaxied layers. A temperature-magnetic field phase diagram of a terbium film is presented.