Macronutrients profoundly affect both lifespan and reproduction and also modulate the fundamental trade-off between these two components of fitness in many insects. Beetles represent the largest group of insects, but nutritional interventions in lifespan and reproduction have never been thoroughly explored in this taxon. Here, we used nutritional landscape methodology to determine the effects of protein and carbohydrate intake on lifespan and reproduction in the mealworm beetle, Tenebrio molitor (Coleoptera: Tenebrionidae). The time of death and last reproduction, the number of eggs laid and protein-carbohydrate intake were recorded from beetles maintained on one of 35 chemically defined foods varying in protein-to-carbohydrate ratio (P:C=0:1, 1:5, 1:2, 1:1, 2:1, 5:1 or 1:0) and in protein plus carbohydrate concentration (P+C=25.2%, 33.6%, 42%, 50.4% or 58.8%). Lifespan and reproductive traits increased with higher caloric intake, but their respective trait maxima occurred at different P:C ratios. Female reproductive traits peaked at higher P:C ratios (reproductive lifespan 1:1.06; lifetime egg production 1.31:1; egg production rate 1.75:1) than those maximizing lifespan (male lifespan 1:1.38; female lifespan 1:1.36). This divergence indicates a nutrient-mediated trade-off between lifespan and reproduction in this species. Despite this, the nutritional conflict in T. molitor appeared to be less pronounced than what has been observed in other species commonly used in insect ageing research. When given a food choice, T. molitor beetles selected a P:C ratio close to 1:1, which simultaneously supported extended lifespan and high reproductive output.