The electrostatic potential generated by neutral-beam-injection (NBI) heating in a tokamak plasma is investigated using numerical simulations. The density distribution of the NBI fast ions in an assumed tokamak is evaluated using the GNET drift-kinetic-equation solver which is based on the Monte Carlo method. The electrostatic potential is evaluated assuming an adiabatic response of the electrons to the fast-ion density distribution in the plasma. It is found that an electrostatic potential peak is generated near the beam-injection point owing to the trapped fast ions satisfying the zero-precession condition. An analytic model expressing the expected potential except for the peak is derived and shows a good agreement with the radial distribution and linear dependence on the electron temperature predicted by the simulation within a factor of 1–2. The existence of three-dimensional electrostatic trapping may break the poloidally-closed particle orbits, and may change the spatial distribution and transport of high-Z impurity ions.