The nutritional quality of insects is related to many factors, including their rearing conditions. In this study, the effects of temperature on the contents of crude protein, lipids, ash, and amino acids and the body size and weight of Tenebrio molitor larvae were analysed. The larvae were reared with the occurrence of the first 20 pupae in a laboratory incubator at temperatures of 22, 25, and 28 °C. The results revealed that the weight (from 0.09 to 0.15 g), dry matter (DM) content (from 30.72 to 36.55 g/100 g), and fat concentration (from 22.46 to 36.01 g/100 g DM) of the larvae increased with increasing rearing temperature. In contrast, the crude protein content significantly decreased (from 64.33 to 54.41 g/100 g DM). Methionine was the limiting amino acid. The essential amino acid index ranged from 37% to 45%. Information about the effect of temperature on the growth and nutritional parameters of mealworms may contribute to the optimisation of mealworm-rearing technology.