Abstract It is known that the autophagy‐related protein 1 (ATG1) plays critical roles in the regulation of autophagy in mammals and yeast, whereas the function of ATG1 in lepidopteran insects is not well elucidated. Here Spodoptera litura ATG1 (SlATG1) and its interactions with other ATG proteins were characterized. Alternative splicing of SlAtg1 produced at least four transcript variants. Over‐expression and RNA interference knockdown of SlAtg1 demonstrated that SlATG1 enhanced autophagy. SlATG1A‐Green fluorescent protein (GFP) tagged localized in the cytoplasm and formed some punctuate dots, which were colocalized with red fluorescent protein mCherry tagged Spodoptera exigua ATG5 (SeATG5). SlATG1A‐GFP over‐expression reduced the nuclear abundance of mCherry‐SeATG5 but increased its cytoplasmic abundance. Pull‐down, co‐immunoprecipitation and bimolecular fluorescence complementation assays showed that SlATG1A bound to SeATG5 through the N‐terminus of SlATG1A. The over‐expression of FLAG epitope tagged SlATG1A significantly increased the accumulation of the cleaved GFP from GFP‐SeATG5, suggesting the enhanced degradation of GFP‐SeATG5. In addition, we confirmed that the interactions of SlATG1 with other autophagy‐related proteins were conserved. These results provide the first evidence that ATG1 interacts with ATG5 and enhances its degradation in lepidopteran insect cells, which may have important physiological functions.