Camphorsulfonic acid, sometimes abbreviated CSA or 10-CSA is an organosulfur compound.

Like typical sulfonic acids, it is a relatively strong acid that is a colorless solid at room temperature and is soluble in water and a wide variety of organic substances.

It can be prepared by sulfonation of camphor with sulfuric acid and acetic anhydride:[1] Although this reaction appears to be a sulfonation of an unactivated methyl group, the actual mechanism is believed to involve a retro-semipinacol rearrangement, deprotonation next to the tertiary carbocation to form an alkene, sulfonation of the alkene intermediate, and finally, semipinacol rearrangement to re-establish the ketone function.

[2] In organic synthesis, CSA and its derivatives can be used as resolving agents for chiral amines and other cations.

[6] Camphorsulfonic acid is used in some pharmaceutical formulations, where is it referred to as camsilate or camsylate, including trimetaphan camsilate and lanabecestat camsylate.