Two‐dimensional numerical plasma simulations have been carried out in a uniform magnetic field to study the effects of neutral beam injection on plasma diffusion. Neutral beams injected across a magnetic field are assumed to be ionized by various ionization processes in a plasma. It is found that the suprathermal convective motion of a plasma generated by the injection of neutral beams is dissipated via anomalous viscosity, leading to enhanced cross‐field diffusion. The diffusion coefficient depends weakly on the magnetic field and plasma density, similar to the diffusion caused by thermally excited convective cells. The magnitude of the diffusion increases with the injection energy and is much larger than the thermal diffusion because of the presence of suprathermal plasma convection. It is shown that a similar anomalous plasma diffusion may occur in a plasma subject to radio frequency (rf) wave heating where only a localized region of a plasma across the magnetic field is heated to a temperature much higher than the surrounding temperature. Theoretical investigations are described on the scaling of enhanced plasma diffusion.