A thiol, also known as a thio alcohol or mercaptan, is an organic compound that has a thiol (–SH) group bound to a carbon atom. Thiols (general formula RSH) are the sulfur analogs of alcohols. They have unpleasant odors resembling rotten cabbage or garlic. Ethane thiol is a typical thiol, having the formula C₂H₅SH.

Thiols are prepared by excess NaSH of H₂S on RX, sulfur on Grignard reagents, of hydrolysis of thiol esters RSC(O)R¹. Thiols are readily oxidized to disulfides RSSR and substituted to sulfides RSR¹.