Diclofenac (marketed as Voltaren®, Voltarol®, Diclon®, Dicloflex® Difen' and Cataflam'''®) is a non-steroidal anti-inflammatory drug (NSAID) taken to reduce inflammation and an analgesic reducing pain in conditions such as in arthritis or acute injury. It can also be used to reduce menstrual pain.

In the United Kingdom and the United States it may be supplied as either the sodium or potassium salt, while in some other countries only as the potassium salt. Diclofenac is available as a generic drug in a number of formulations. Over the counter (OTC) use is approved in some countries for minor aches and pains and fever associated with common infections.

Diclofenac is well-tolerated after 30 years experience by the general human population, but may unexpectedly become intolerated in some of the elderly population of long term users.