This study explored the synthesis of activated carbon from olive seeds by chemical and thermal activation, and evaluated the energy efficiency of the process. The oilseed samples were carbonized at varying temperature (600–840 °C). The produced activated carbons were used for dye removal, studying the effect of temperature. The materials were analyzed using X-ray diffraction and scanning electron microscopy (SEM). The pores and interpore spacing porous of the activated carbons were identified from the SEM images using ImageJ software. The results show that the yield of activated carbon and energy efficiency significantly decreased with increasing carbonization temperature. This reveals that a higher yield of activated carbon and higher energy efficiency was attained at lower carbonization temperature, meaning that producing activated carbon from olive seed is cost effective. Moreover, the activated carbon obtained at 600 °C exhibits the optimum removal efficiency for methylene blue, however, the increase in the removal efficiency upon increasing the temperature from 600 to 720 °C is insignificant (about 1%).