The present invention provides a sulfonium salt used in a resist composition that can give a pattern having a high resolution, especially an excellent rectangularity of a pattern form and a small roughness, while not readily generating a defect, in the photolithography using a high energy beam as a light source; a resist composition that contains the sulfonium salt; and a patterning process using this resist composition, wherein the sulfonium salt is shown by the following general formula (1a),
wherein each of R and R0 independently represents a hydrogen atom, or a linear, branched, or cyclic monovalent hydrocarbon group having 1 to 30 carbon atoms which may be optionally substituted by a heteroatom or interposed by a heteroatom.