Macroautophagy (hereinafter called autophagy) is a highly regulated process used by eukaryotic cells to digest portions of the cytoplasm that remodels and recycles nutrients and disposes of unwanted cytoplasmic constituents. Currently 36 autophagy-related genes (ATG) and their homologs have been characterized in yeast and higher eukaryotes, including insects. In the present study, we identified and functionally characterized the immune function of an ATG8 homolog in a coleopteran insect, Tenebrio molitor (TmATG8). The cDNA of TmATG8 comprises of an ORF of 363 bp that encodes a protein of 120 amino acid residues. TmATG8 transcripts are detected in all the developmental stages analyzed. TmAtg8 protein contains a highly conserved C-terminal glycine residue (Gly116) and shows high amino acid sequence identity (98%) to its Tribolium castaneum homolog, TcAtg8. Loss of function of TmATG8 by RNAi led to a significant increase in the mortality rates of T. molitor larvae against Listeria monocytogenes. Unlike dsEGFP-treated control larvae, TmATG8-silenced larvae failed to turn-on autophagy in hemocytes after injection with L. monocytogenes. These data suggest that TmATG8 play a role in mediating autophagy-based clearance of Listeria in T. molitor.