The yellow mealworm, Tenebrio molitor L. (Coleoptera: Tenebrionidae), is an important industrial insect commercially produced around the world as food and feed. Temperature and nutrition are the two most influential environmental factors determining the rearing conditions in insects, but little is known about how these two factors interact to affect the performance of T. molitor larvae. In this study, we investigated the combined effects of temperature and dietary protein:carbohydrate (P:C) ratio on key performance traits in T. moltior larvae. Throughout their larval stage, the insects were reared on one of 36 treatment combinations of six temperatures (19, 22, 25, 28, 31, 34 °C) and six protein:carbohydrate ratios (P:C = 1:5, 1:2, 1:1, 2:1, 5:1, 1:0) and their survivorship, development, growth rate, and pupal mass were monitored. Survivorship was high at low temperatures (< 25°C) and high P:C ratios (>1:1), but decreased with increasing temperature and decreasing P:C ratio. Increase in rearing temperature accelerated larval development but resulted in a reduced pupal mass. Thermal optimum for pupal mass (19.3°C) was thus lower than that for development time (28.1°C). The growth rate was maximized at 27.9°C and P:C 1.65:1 and decreased as both the temperature and the P:C ratio deviated from their optimum. All four key performance traits (survivorship, development time, pupal mass, growth rate) were optimized at temperatures between 25.7 and 27.4°C and P:C ratios between 1.17:1 and 2.94:1. Our data provide insights into how the production and nutritional value of T. molitor larvae can be improved through adjusting their rearing conditions.