The hallmark of cellular events observed upon macroautophagic/autophagic induction is the conjugation of LC3B, one of the mammalian Atg8 homologs, with phosphatidylethanolamine. This conversion from LC3B-I (an unconjugated form) to LC3B-II (a conjugated form) is essential for phagophore expansion and formation of autophagosomes. Our recent study revealed that LC3B binds to RNAs with a preference for the consensus AAUAAA motif and recruits the CCR4-NOT deadenylase complex. Consequently, LC3B elicits rapid degradation of mRNAs, which we have termed as LC3B-mediated mRNA decay (LMD). LMD requires the conversion of LC3B-I to LC3B-II and occurs before the formation of autolysosomes. Furthermore, we identified PRMT1 mRNA, which encodes a protein that functions as a negative regulator of autophagy, as an LMD substrate. A failure of rapid degradation of PRMT1 mRNA via LMD results in inefficient autophagy. Thus, our study unravels an important role of LC3B in autophagy as an RNA-binding protein for efficient mRNA decay.