The yellow mealworm, Tenebrio molitor L., is one of the most important industrialized insects worldwide. As a poikilotherm, T. molitor requires a temperature-regulated environment for its farming. Currently, the farming of T. molitor uses temperatures between 29 and 31°C determined to be optimal for maximizing growth. This study was aimed to test and determine optimal temperature for T. molitor impacting all stages including their development, reproduction, and fitness. Pupae of T. molitor were exposed to 8 different temperatures (18, 21, 24, 27, 30, 33, 36, and 39°C). Developmental rates (=1/d to develop) were calculated at 8 temperatures and fitted in a poikilotherm model to determine the temperature with the highest developmental rate (developmental maximum, T_o).The developmental maximum was 33°C and the thermal maximum (t_m) (lethal temperature) was 39°C. The biological and demographic parameters of T. molitor were compared at 3 favorable temperatures below the T_o (31, 29, and 27°C) to determine the fitness of T. molitor populations. Fecundity and egg viability were the highest at 27°C (5.25 eggs/female/d and 73.99%, respectively) and the lowest at 31°C (1.55 eggs/ female/d and 27.14%, respectively).The shortest larval development time was observed at 27°C and the longest at 31°C (142.81 and 161.97 d, respectively).The highest fitness was observed at 27°C with a population doubling time of 32.78 d compared to 40.58 and 239.4 d at 29 and 31°C, respectively. In conclusion, 27°C is closer to the optimal temperature for T. molitor productivity than 29°C, thus reducing its farming energy needs.