A positive resist composition comprising (A) a compound capable of generating an acid upon irradiation with an actinic ray or radiation, (B) a resin capable of increasing the solubility in an alkali developer by the action of an acid, and (C) a compound having a specific structure, which decomposes by the action of an acid to generate an acid, a pattern forming method using the positive resist composition, and a compound for use in the positive resist composition are provided as a positive resist composition exhibiting good performance in terms of pattern profile, line edge roughness, pattern collapse, sensitivity and resolution in normal exposure (dry exposure), immersion exposure and double exposure, a pattern forming method using the positive resist composition and a compound for use in the positive resist composition.