A plate surface treatment agent for a lithographic printing plate is provided that includes a vinyl copolymer containing a monomer unit having at least one group selected from the group consisting of a phosphonic acid group, a phosphoric acid group, a carboxylic acid group, and a salt of these groups and a monomer unit having at least one group or structure selected from the group consisting of a sulfonic acid group, a salt thereof, an amide group, and a betaine structure. There is also provided a method for treating a lithographic printing plate that includes a step of imagewise exposing a lithographic printing plate precursor, a step of processing using a developer, and a step of carrying out a plate surface treatment using the plate surface treatment agent for a lithographic printing plate.