Abstract Multiphase flow and mineral reactions co‐exist in many subsurface systems, and thus elucidating and quantifying the impacts of multiphase flow on mineral reaction rates are crucial. Our study established a benchmark experiment on calcite dissolution in cylindrical pores in slug flow, and performed a series of simulations using a validated pore‐scale multiphase reactive transport model. The results revealed contrasting impacts of the two‐phase flow on transport in the aqueous phase and the effective surface area, and that the relative contribution of these factors determines whether the bulk reaction rate in two‐phase flow is lower (regime I) or higher (regime II) than that in single‐phase flow. Scaling laws were attempted to relate reaction rate with velocity to account for the enhanced transport by internal circulations, and variability in the effective surface area was also discussed. This study provides useful insights for upscaling mineral reaction rate in two‐phase flow in porous media.