Solvothermal synthesis, denoting chemical reactions occurring in metastable liquids above their boiling point, normally requires the use of a sealed autoclave under pressure to prevent the solvent from boiling. This work introduces an experimental approach that enables solvothermal synthesis at ambient pressure in an open reaction medium. The approach is based on the use of gold nanoparticles deposited on a glass substrate and acting as photothermal sources. To illustrate the approach, the selected hydrothermal reaction involves the formation of indium hydroxide microcrystals favored at 200 °C in liquid water. In addition to demonstrating the principle, the benefits and the specific characteristics of such an approach are investigated, in particular, the much faster reaction rate, the achievable spatial and time scales, the effect of microscale temperature gradients, the effect of the size of the heated area, and the effect of thermal-induced microscale fluid convection. This technique is general and could be used to spatially control the deposition of virtually any material for which a solvothermal synthesis exists.