Turbulent mixing flows of supercritical water and a metal-salt solution were investigated using Reynolds-averaged Navier–Stokes (RANS) simulations. The mass conservation equations for metal-salt and metal-oxide in an aqueous solution, which were coupled with Navier–Stokes equations and the Shear Stress Transport (SST) turbulence model, were solved by considering production by the hydrothermal reaction. The reaction rate in the numerical simulation was interpolated linearly using the experimental data. The mixing flows in a T-shaped channel for various Reynolds numbers were simulated numerically. Fluid mixing causes a hydrothermal reaction in a high temperature region. In a situation with a low temperature and low Reynolds number, the mixing became a steady state, and the metal oxide was generated along the channel wall. For a high Reynolds number, the periodic vortexes were observed at the mixing point and the fluid temperature increased rapidly. A numerical simulation reproduced the apparent reaction rate of the experimental measurements, except for the low Reynolds number case. The time-averaged temperature distributions indicated that the increasing temperature rate in the mixing reactor depends on the inlet supercritical water temperature, which affects the distribution of the concentration of metal oxide. If the turbulence effects were ignored in low-temperature instances, the apparent reaction rate was estimated to be quite low. The turbulent diffusivity and thermal conductivity crucially affected the conversion rate, especially for conditions with a low Reynolds number.