The online encyclopedia's English language website is down until midnight. The 24-hour blackout is to protest anti-piracy legislation under consideration in Congress.
The Stop Online Piracy Act and the Protect Intellectual Property Act are designed to crack down on sales of pirated U.S. products overseas.
Supporters including the film and music industry say the legislation is needed to protect intellectual property and jobs.
Critics say it could hurt the technology industry and infringe on free-speech rights. Tech companies such as Google, Facebook, Yahoo, Twitter, eBay, AOL and others have said it threatens the industry's livelihood.
Several online communities such as Reddit, Boing Boing and others have announced plans to go dark in protest as well.