We report a light-induced, surface-assisted structural phase transition from a common orthorhombic phase of crystalline gallium (α-gallium) to a highly reflective, metastable phase of more ‘metallic’ nature. The transition has been observed at the interface of gallium with fused silica at temperatures just below the metal's bulk melting point and affects only several tens of gallium atomic layers. The transition is fully reversible and occurs on a nanosecond/microsecond time scale. The transition appears to show some characteristic features of a second order structural phase transition, including an increase of the transition relaxation times at the critical temperature (of approximately 30°C). The transition has no apparent optical intensity threshold, and is induced by radiation of very low intensity of only 10−4 − 10−5 W/μm2. The two gallium phases involved in the phase transition have significantly different dielectric properties which gives rise to a gigantic cubic optical nonlinearity, χ(3) ∼ 1 esu. The transition can be stimulated by light at any wavelength in the visible and the infrared ranges out to at least 1.55μm. The effect is therefore of great interest for applications requiring light by light control at milliwatt power levels.