[Problem] To provide: a water-based hydrophilizing surface treatment agent for forming a hydrophilic coating film that exhibits excellent hydrophilicity, hydrophilicity sustainability and corrosion resistance; a hydrophilic coating film; and a hydrophilizing surface treatment method. [Solution] The above-mentioned problem is solved by a water-based surface treatment agent containing an organic-inorganic silicon compound (A) and inorganic particle (B), wherein the organic-inorganic silicon compound (A) is adapted to be a compound that is obtained by mixing a colloidal silica (C) and an organoalkoxysilane (D). In this regard, the ratio (M B /M A ) between the solid content mass M A of the organic-inorganic silicon compound (A) and the solid content mass M B of the inorganic particle (B) preferably falls within the range from 0.2 or more and 2. 0 or less, the average particle size of the organic-inorganic silicon compound (A) preferably falls within the range of 3 nm or more and 500 nm or less. In addition, th
