Anatase　TiO2　quantum　dots　(QDs)　with　surface　hydroxyl　groups　and　particle　size　below　3　nm　were　synthesized　via　a　new　synthetic　route　and　showed　a　low　band-gap　energy　of　2.85　eV　and　good　dispersibility　in　an　aqueous　system.　This　material　exhibited　significantly　high　photocatalytic　activity　for　the　degradation　of　methyl　orange　(MO).　The　photocatalytic　activity　of　TiO2　QDs　was　16　times　higher　than　that　of　commercial　P25　at　an　initial　MO　concentration　of　50　mg　L–1　in　the　reaction　system.　At　a　concentration　of　10　mg　L–1,　MO　could　almost　be　completely　removed　within　1　min.　The　high　photocatalytic　activity　could　be　attributed　to　the　low　band-gap　energy,　large　surface　area,　and　good　aqueous　dispersibility　of　the　anatase　TiO2　QDs.　©　2018　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim