The problem to be solved by the present invention is to provide an aqueous coating composition that can form a multilayer coating film having excellent storage stability, excellent adhesion to plastic base materials, excellent finish, and excellent coating film performance, such as water resistance; and to also provide a coating method using the composition. The present invention provides an aqueous coating composition comprising (A) an aqueous dispersion of a modified polyolefin, (B) an aqueous acrylic resin and/or an aqueous polyester resin, and (C) a phosphoric acid compound represented by a specific formula; and also provides a method for forming a coating film, comprising applying the aqueous coating composition, as well as an article coated with the aqueous coating composition.