A combined scanning tunneling microscopy and low-energy electron diffraction investigation of the Mn/GaAs(001) interface formation is reported. The interface, grown on a (2 × 4) reconstructed substrate produced by molecular beam epitaxy, was studied as a function of Mn evaporation with thickness ranging from 1/8 ML to 1 ML. The interaction of Mn atoms with the semiconductor surface is strong and leads to surface reconstructions involving a rearrangement of the two outmost atomic layers of the substrate. For Mn thickness lower than 1/2 ML, the surface is characterized by a (2 × 1) periodicity. Conversely, when the Mn deposition is increased to 1/2 ML the surface reconstruction is strongly dependent on the preparation procedure. If Mn deposition is performed on the substrate at 390 °C, a fully ordered surface characterized by a clear (2 × 2) reconstruction is obtained, whereas, annealing the sample after Mn deposition, gives a disordered surface with a (2 × 1) symmetry. An intermediate phase between (2 × 1) and (2 × 2) is found for Mn depositions in between 1/4 and 1/2 ML. No further structural evolution was observed for both preparation methods above 1/2 ML coverage.