The PAR‐domain protein 1 (PDP1) is essential for locomotor activity of insects. However, its functions in insect growth and development have not been studied extensively, which prompted our hypothesis that PDP1 acts in energy metabolism. Here we report identification of TcPDP1 in the red flour beetle, Tribolium castaneum, and its functional analysis by RNAi. Treating larvae with dsTcPDP1 induced pupae developmental arrestment, accompanied by accelerated fat body degradation. dsTcPDP1 treatments in adults resulted in reduced female fecundity. Disruption of TcPDP1 expression affected the transcription of genes involved in insulin signaling transduction and mechanistic target of rapamycin (mTOR) pathway. These results support our hypothesis that TcPDP1 acts in energy metabolism in T. castaneum.