In organic chemistry, a toluenesulfonyl group (tosyl group, abbreviated Ts or Tos) is a univalent functional group with the chemical formula −SO₂−C₆H₄−CH₃. It consists of a tolyl group, −C₆H₄−CH₃, joined to a sulfonyl group, −SO₂−, with the open valence on sulfur. This group is usually derived from the compound tosyl chloride, CH₃C₆H₄SO₂Cl (abbreviated TsCl), which forms esters and amides of toluenesulfonic acid, CH₃C₆H₄SO₂OH (abbreviated TsOH). The para orientation illustrated (p-toluenesulfonyl) is most common, and by convention tosyl without a prefix refers to the p-toluenesulfonyl group.

The toluenesulfonate (or tosylate) group refers to the −O−SO₂C₆H₄CH₃ (–OTs) group, with an additional oxygen attached to sulfur and open valence on an oxygen. In a chemical name, the term tosylate may either refer to the salts containing the anion of p-toluenesulfonic acid, TsO⁻M⁺ (e.g., sodium p-toluenesulfonate), or it may refer to esters of p-toluenesulfonic acid, TsOR (R = organyl group).