The process for producing an olefin dimer of the present invention includes a first step of carrying out a dimerization reaction of an olefin in the presence of a solid phosphoric acid catalyst in which phosphoric acid is supported on inorganic support particles at a reaction temperature of 55 to 300° C. by introducing into a reactor an olefin-containing raw material containing water in an amount of 10 ppm by mass or more and less than the saturated water content at the reaction temperature, thereby preparing a reaction product containing an olefin dimer, a second step of washing the reaction product prepared in the first step at a temperature of 50° C. or higher using an alkaline substance-containing water adjusted to pH 8 to 13 and a third step of washing the reaction product after the alkaline washing in the second step with water at a temperature of 0 to 110° C., thereby preparing an olefin dimer.