Diclofenac, sold under the brand name Voltaren, among others, is a nonsteroidal anti-inflammatory drug (NSAID) used to treat pain and inflammatory diseases such as gout. It is taken by mouth or rectally in a suppository, used by injection, or applied to the skin. Improvements in pain last for as much as eight hours. It is also available in combination with misoprostol in an effort to decrease stomach problems.

Common side effects include abdominal pain, gastrointestinal bleeding, nausea, dizziness, headache, and swelling. Serious side effects may include heart disease, stroke, kidney problems, and stomach ulceration. Use is not recommended in the third trimester of pregnancy. It is likely safe during breastfeeding. Diclofenac is believed to work by decreasing the production of prostaglandins, like other drugs in this class.

Diclofenac was patented in 1965 by J.R. Geigy AG; it came into medical use in the United States in 1988. It is available as a generic medication. In 2020, it was the 72nd most commonly prescribed medication in the United States, with more than 9 million prescriptions. It is available as a sodium or potassium salt. It is also widely used for livestock; such use was responsible for the Indian vulture crisis, during which in a few years 95% of the country's vulture population was killed, and in many countries agricultural use is now forbidden.