The present invention provides a polishing composition for a glass substrate having a pH of from 1 to 5 and containing silica particles having an average particle size of from 5 to 100 nm, wherein, in a projected image of the silica particles obtainable by an image analysis of electron photomicrographs, an average of an area ratio R of a projected area of the silica particles (A1) to an area of a maximum inscribed circle of the silica particles (A), i.e. (A1/A), is in the range of from 1.2 to 3.0, and the silica particles have an average of 2.0 to 10 projection portions having a curvature radius of from 1/5 to 1/2 of a radius of the maximum inscribed circle of the silica particles (r) on the outline of the silica particles in the projected image, and a method for manufacturing a glass substrate, including the step of polishing a substrate to be polished with a polishing load of from 3 to 12 kPa and at a pH of from 1 to 5 while allowing the polishing composition to be present between a polishing pad and the substrate to be polished. The polishing composition for a glass substrate of the present invention can be suitably used, for example, in the manufacture of glass hard disks, aluminosilicate glass for reinforced glass substrates, glass ceramic substrates (crystallized glass substrates), and the like.