The phosphinositide Ptdlns(3)P plays an important role in autophagy; however, the detailed mechanism of its activity remains unclear. Here, we used a Systematic Evolution of Ligands by EXponential enrichment (SELEX) screening approach to identify an RNA aptamer of 40 nucleotides that specifically recognizes and binds to intracellular lysosomal Ptdlns(3)P. Binding occurs in a magnesium concentration- and pH-dependent manner, and consequently inhibits autophagy as determined by LC3II/I conversion, p62 degradation, formation of LC3 puncta, and lysosomal accumulation of Phafin2. These effects in turn inhibited lysosomal acidification, and the subsequent hydrolytic activity of cathepsin D following induction of autophagy. Given the essential role of Ptdlns(3)P as a key targeting molecule for autophagy induction, identification of this novel Ptdlns(3)P RNA aptamer provides new opportunities for investigating the biological functions and mechanisms of phosphoinositides. (C) 2019 The Authors. Published by Elsevier Inc.