The effect of size reduction on the phase diagrams of metallic alloys is of great importance for modern applications. Gallium-based alloys are considered as prospective nanomaterials for soft robotics and wearable electronics. Here, we present NMR studies of the Ga–In–Sn eutectic alloy confined to a porous glass with pore size 18 nm, which demonstrated the liquid–liquid phase transition in the supercooled state of the alloy. The phase transition was detected upon cooling by emergence of a second component of the ⁷¹Ga NMR line with higher Knight shift. This additional component survived at lower temperatures and was observed upon successive warming. ¹¹⁵In NMR measurements were also conducted to monitor the alloy composition at freezing and melting. The critical point was evidenced. The splitting of the ⁷¹Ga NMR line was not found in a binary Ga–Sn alloy embedded into porous glass and opal templates, which suggests that liquid–liquid phase transition is sensitive to the alloy composition.