Modafinil (INN, USAN, BAN, JAN) is a wakefulness-promoting agent (or eugeroic) used for treatment of disorders such as narcolepsy, shift work sleep disorder, and excessive daytime sleepiness associated with obstructive sleep apnea. It has also seen widespread off-label use as a purported cognition-enhancing agent. In English-speaking countries it is sold under the brand names Alertec, Modavigil, and Provigil. In the United States modafinil is classified as a schedule IV controlled substance and restricted in availability and usage, due to concerns about possible addiction potential. In most other countries it is a prescription drug but not otherwise legally restricted.

Although the mechanism of action of modafinil was initially unknown, it now appears that the drug acts as a selective, relatively weak, atypical dopamine reuptake inhibitor. However, it appears that other additional mechanisms may also be at play.