Provided are a photoacid generator, a photoresist composition including the same, and a method of forming a pattern by using the photoacid generator. The photoacid generator includes a copolymer of a monomer that generates an acid upon exposure to light and an acid-labile monomer of which solubility with respect to a developing solvent is changed by decomposition by an acid, wherein the copolymer is represented by Formula 1:






wherein, in Formula 1, x, y, L, A−, B+, R1, R2, and R3 are each the same as described in the detailed description.