A novel sulfonium salt having high sensitivity with respect to active energy rays, a photoacid generator including the sulfonium salt, and a photosensitive composition containing the photoacid generator. The sulfonium salt is represented by formula (a1). In the formula, R1 and R2 each independently represent the group that is represented by formula (a2) or an alkyl group that may be substituted by a halogen atom, R1 and R2 are bonded to each other and may form a ring with the sulfur atom within the formula, R3 is the group represented by formula (a3) or the group represented by formula (a4), A1 represents S or the like, X− represents a monovalent anion, and R1 and R2 are not both an alkyl group which may be substituted with a halogen atom. In formulas (a2) to (a4), the ring Z1 represents an aromatic hydrocarbon ring, R4, R6, R9, and R10 each represents a specific monovalent group, R5, R7, and R8 each represents a specific divalent group, A2 and A3 each represents S or the like, m1 represents an integer of 0 or more, and n1 and n2 each represent 0 or more.