甲磺酸（英语：methanesulfonic acid）是最简单的烷基磺酸。

无色液体。易溶于水，微溶于有机溶剂。具强酸性，pKa为-0.6。在热水或热碱液中不水解。

Methanesulfonic acid (MsOH) is a colorless liquid with the chemical formula CH₃SO₃H. It is the simplest of the alkylsulfonic acids. Salts and esters of methanesulfonic acid are known as mesylates (or methanesulfonates, as in ethyl methanesulfonate). It is hygroscopic in its concentrated form. Methanesulfonic acid may be considered an intermediate compound between sulfuric acid (H₂SO₄), and methylsulfonylmethane ((CH₃)₂SO₂), effectively replacing an –OH group with a –CH₃ group at each step. This pattern can extend no further in either direction without breaking down the –SO₂– group. Methanesulfonic acid can dissolve a wide range of metal salts, many of them in significantly higher concentrations than in hydrochloric or sulphuric acid.