Tenebrio molitor (Coleoptera: Tenebrionidae) is a suitable candidate for use as a biomass resource, recognized for its large-scale breeding and the high nutritional value of its protein content. Feeding obese Zucker rats the cuticle of T. molitor enhances fatty liver metabolism through the mediation of gut microorganisms. Cuticular proteins are demonstrated to be pivotal in the formation of the insect cuticle throughout the developmental stage. The endocuticle structural glycoprotein (Abd) belongs to the RR-1 subclass, a major group of structural cuticular proteins characterized by the conserved Rebers–Riddiford (RR) motif. Nevertheless, there remains a paucity of research into the molecular properties and functions of SgAbd (endocuticle structural glycoprotein) in Coleoptera. In this study, we successfully identified and described the gene TmAbd5 in T. molitor. The coding sequence of TmAbd5 is 306 bp, corresponding to a 101-aa protein. The functional domain predicted that TmAbd5 consists of a signal peptide and a chitin-binding domain 4 (ChtBD4). Motif prediction analysis indicated that TmAbd5 belongs to the CPR (cuticular proteins with Rebers–Riddiford consensus) family with the RR-1 motif. Expression analysis revealed that TmAbd5 is upregulated in the integument, particularly during the first three days of development in the 13th instar stage. Although the RNAi-mediated silencing of TmAbd5 did not cause apparent phenotypic abnormalities and the insects successfully molted into pupae, histological examination revealed a substantial thickening of the endocuticle at 72 h post-pupation, along with a notable increase in lamellar spacing and a disrupted pore canal. In summary, TmAbd5 contributes to the formation and structural organization of the endocuticle, which provides a theoretical basis for the screening of target genes for cuticle development and for the effective utilization of cuticle resources in T. molitor.