Macroautophagy is a bulk degradation system conserved in all eukaryotic cells. A ubiquitin-like protein, Atg8, and its homologues are essential for autophagosome formation and act as a landmark for selective autophagy of aggregated proteins and damaged organelles. In this study, we report evidence demonstrating that OATL1, a putative Rab guanosine triphosphatase–activating protein (GAP), is a novel binding partner of Atg8 homologues in mammalian cells. OATL1 is recruited to isolation membranes and autophagosomes through direct interaction with Atg8 homologues and is involved in the fusion between autophagosomes and lysosomes through its GAP activity. We further provide evidence that Rab33B, an Atg16L1-binding protein, is a target substrate of OATL1 and is involved in the fusion between autophagosomes and lysosomes, the same as OATL1. Because both its GAP activity and its Atg8 homologue–binding activity are required for OATL1 to function, we propose a model that OATL1 uses Atg8 homologues as a scaffold to exert its GAP activity and to regulate autophagosomal maturation.