Fluids within the Earth's crust may exist under supercritical conditions (i.e., >374°C and >22.1 MPa for pure water). Supercritical geothermal reservoirs at depths of 2–10 km below the surface in northeastern (NE) Japan mainly consist of magmatic fluids that exsolved from the melt during the course of fractional crystallization. Supercritical geothermal reservoirs have received attention as next‐generation geothermal resources because they can offer significantly more energy than that obtained from conventional geothermal reservoirs found at temperatures <350°C. However, the spatial distribution and fluid fraction of supercritical geothermal reservoirs, which are required for their resource assessment, are poorly understood. Here, the magnetotelluric (MT) method with electrical resistivity imaging is used in the Yuzawa geothermal field, NE Japan, to collect data on the fluid fraction and spatial distribution of a supercritical geothermal reservoir. The collected MT data reveal a potential supercritical geothermal reservoir (>400°C) with dimensions of 3 km (width) × 5 km (length) at a depth of 2.5–6.0 km below the surface. The estimated fluid fraction of the reservoir is 0.1%–4.2% with salinity values of 5–10 wt%. The melt is also imaged below the reservoir, and based on the resistivity model; we develop a mechanism for the evolution of the supercritical geothermal reservoir, wherein upwelling supercritical fluids supplied from the melt are trapped under less permeable silica sealing and accumulate there.