A coating agent for forming a grain-oriented electrical steel sheet coating able to form an aluminum borate coating high in adhesion and large in tension and a method for manufacturing the grain-oriented electrical steel sheet are provided. The coating agent for forming a grain-oriented electrical steel sheet coating of the present invention comprises an aluminum source containing aluminum oxide and/or an aluminum oxide precursor compound, a boron source containing a borate of an alkali metal, and silicon oxide and/or a silicon oxide precursor in an amount, converted to silicon oxide, of 5 mass% or more and 10 mass% or less with respect to a total solids concentration of the aluminum source and boron source, the aluminum source and the boron source contained so that, by molar ratio, Al/B: 0.5 to 2.0, a solids concentration of a total of the aluminum source and the boron source being 20 mass% or more and 38 mass% or less, and pH being 2.0 or more and 6.0 or less.