The move is likely to further worry rights groups and Turkey's Western allies, who say Ankara has sharply curtailed freedom of speech and other basic rights in the crackdown that followed last year's failed coup.
"After technical analysis and legal consideration... an administrative measure has been taken for this website (Wikipedia.Org)," the BTK telecommunications watchdog said in a statement on its website.
It cited a law that allows it to block access to individual web pages or entire websites for the protection of public order, national security or the well-being of the public.
Turkey's communications ministry said Wikipedia was attempting to run a "smear campaign" against Turkey, saying some articles purported that Ankara was coordinating with militant groups, state-run Anadolu news agency reported.
"Instead of coordinating against terrorism, it has become part of an information source which is running a smear campaign against Turkey in the international arena," Anadolu quoted the ministry as saying in a statement.
The ban would be lifted if Wikipedia met the government's demands, Anadolu said.
Under the law, the watchdog is required to submit its ban to a court within 24 hours. The court then has two days to decide whether the ban should be upheld.
'CENSOR CONTENT'
A block on all language editions of the Wikipedia website was detected at 8:00 am on Saturday, monitoring group Turkey Blocks said on its website.
"The loss of availability is consistent with internet filters used to censor content in the country," it said.
When attempting to access the web page using Turkish internet providers, users received a notice the site could not be reached and a "connection timed out" error.
Monitoring groups have accused Turkey of blocking access to social media sites such as Twitter or Facebook, particularly in the aftermath of militant attacks.
The government has in the past denied blocking access to some sites, blaming outages on spikes in usage after major events. But technical experts at watchdog groups say the blackouts on social media are intentional, aimed in part at stopping the spread of militant images and propaganda.
Since last year's failed coup, authorities have sacked or suspended more than 120,000 people from the civil service, police and judiciary and arrested more than 40,000 on suspicion of ties to terrorist groups.
President Tayyip Erdogan says the measures are needed given the scope of the security threat Turkey faces.
Turkey last year jailed 81 journalists, making it the world's top jailor of journalists, according to the New York-based Committee to Protect Journalists.