Eddy diffusivity in a planar turbulent jet with a second-order chemical reaction (A+B→R) is investigated by using direct numerical simulation. Reactant A and B are premixed into the jet flow and the main flow, respectively. The eddy diffusivity of species A on the jet centerline is calculated from the mean concentration of species A and the streamwise turbulent mass flux of the species A. The results show that the chemical reaction makes the eddy diffusivity of species A small, and the effects of the chemical reaction on the eddy diffusivity depend on the Damköhler number. From these results, we can conclude that the gradient diffusion model should be used with considerations of the effects of chemical reactions on eddy diffusivity.