Using low-energy electron diffraction and scanning tunnelling microscopy, we studied the formation of Mg silicide and metallic Mg islands on a Si(111)-7 × 7 surface at room temperature as a function of Mg coverage. We found that the mechanism by which Mg islands grew on the Si(111)-7 × 7 surface, and the morphology of the islands that formed, depended on whether the Mg deposition was performed in a stepwise or continuous manner. When Mg was deposited in a stepwise manner, with 1 h between deposition events, an amorphous Mg silicide overlayer formed on the Si(111)-7 × 7 surface during the initial stage of deposition (up to 2.0 ML Mg coverage), as shown by the observation of δ7 × 7 and 1 × 1 low-energy electron diffraction patterns. Upon further stepwise Mg deposition, round-shaped Mg islands grew on the amorphous Mg silicide layer, as shown by scanning tunnelling microscopy and the emergence of a 1 × 1 low-energy electron diffraction pattern. If, on the other hand, the Mg was deposited continuously in a single step, hexagonal Mg islands formed on the flat Mg silicide layers, and a [Formula: see text] and 1 × 1 mixed phase was observed. Moreover, using scanning tunnelling spectroscopy, we confirmed the semiconducting and metallic nature of the Mg silicide layer and hexagonal Mg islands on the Si(111)-7 × 7 surface depending on their Mg coverage, respectively.