Reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) has become a widely used technique to quantify gene expression. It is necessary to select appropriate reference genes for normalization. In the present study, we assessed the expression stability of seven candidate genes in Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) irradiated by ultraviolet B (UVB) at different developmental stages for various irradiation time periods. The algorithms of geNorm, NormFinder, and BestKeeper were applied to determine the stability of these candidate genes. Ribosomal protein genes RpS3, RpL13A, and β-actin gene (ActB) showed the highest stability across all UVB irradiation time points, whereas expression of other normally used reference genes, such as those encoding the β-tubulin gene TUBB and the E-cadherin gene CAD, varied at different developmental stages. This study will potentially provide more suitable reference gene candidates for RT-qPCR analysis in T. castaneum subjected to environmental stresses, particularly UV irradiation.