To provide a surface treatment liquid capable of successfully making a surface of a treatment target hydrophilic or hydrophobic even without including resin having a coating film formation property, and a surface treatment method using the surface treatment liquid. In a surface treatment liquid including (A) resin and a (C) solvent, resin having a functional group I that is at least one group selected from the group consisting of a hydroxyl group, a cyano group, and a carboxyl group, and a functional group II that is a hydrophilic group or a hydrophobic group other than the functional group I is used as the (A) resin, and (B) strong acid having a pKa of 1 or less is added to the surface treatment liquid.