Dapsone, also known as 4,4'-sulfonyldianiline (SDA) or diaminodiphenyl sulfone (DDS), is an antibiotic commonly used in combination with rifampicin and clofazimine for the treatment of leprosy. It is a second-line medication for the treatment and prevention of pneumocystis pneumonia and for the prevention of toxoplasmosis in those who have poor immune function. Additionally, it has been used for acne, dermatitis herpetiformis, and various other skin conditions. Dapsone is available both topically and by mouth.

Severe side effects may include a decrease in blood cells, red blood cell breakdown especially in those with glucose-6-phosphate dehydrogenase deficiency (G-6-PD), or hypersensitivity. Common side effects include nausea and loss of appetite. Other side effects include liver inflammation, methemoglobinemia, and a number of types of skin rashes. While the safety of use during pregnancy is not entirely clear some physicians recommend that it be continued in those with leprosy. It is of the sulfone class.

Dapsone was first studied as an antibiotic in 1937. Its use for leprosy began in 1945. It is on the World Health Organization's List of Essential Medicines. The form, which is taken by mouth, is available as a generic drug and not very expensive.