Turkey's Information and Communication Technologies Authority (BTK) said it had implemented the ban against Wikipedia.org, without making clear the reason for the move.
Turkish state media said the ban was imposed because Wikipedia had failed to remove content promoting terror and accusing Turkey of cooperation with various terror groups.
There was no indication as to when the ban might be removed, with a formal court order expected to follow in the coming days.
Reacting to the ban, Wikipedia's founder Jimmy Wales wrote on Twitter: "Access to information is a fundamental human right. Turkish people, I will always stand with you to fight for this right."
A block affecting all language editions of the website in Turkey was detected from 0500 GMT after an administrative order by the Turkish authorities, according to the Turkey Blocks monitoring group, which follows internet restrictions in the country.
Residents in Istanbul were unable to access any pages of Wikipedia on Saturday morning without using a Virtual Private Network (VPN), AFP correspondents said.
"The loss of availability is consistent with internet filters used to censor content in the country," Turkey Blocks said.
'Law No. 5651'
The BTK confirmed the ban in a statement but gave no details.
"After technical analysis and legal consideration based on the Law No. 5651, an administrative measure has been taken for this website Wikipedia.org," it said.
Law 5651, passed in 2014 by parliament, bolstered the BTK's control over the internet and was seen at the time by freedom of expression activists as an erosion of online liberties.
The incident quickly spawned its own separate Wikipedia entry -- "Wikipedia blocked in Turkey".
Quoting Turkey's transport and communications ministry, the state-run Anadolu news agency said the ban was imposed because Wikipedia had failed to take down content purporting to show Turkey "on the same level as and cooperating with" terror groups.
It said Turkey had kept in contact with Wikipedia but the site had failed to remove the content in question.
No further details were given but Turkey has long taken a hard line against what it calls "terror propaganda" in favour of the outlawed Kurdistan Workers' Party (PKK).
Critics of Turkey, including Kurdish militants, have accused Ankara of occasionally collaborating with jihadists in Syria, a claim fiercely rejected by the government.
'Temporary security measures'
Turkey has become notorious over the last years for temporarily blocking access to popular sites, including Facebook and Twitter, in the wake of major events such as mass protests or terror attacks.
In March 2014, YouTube was banned for several months in Turkey after the site was used to broadcast purported footage of a security meeting on Syria.
In the summer of 2013, severe restrictions were imposed on social media during huge protests against President Recep Tayyip Erdogan, who was prime minister at the time.
Savvy internet users frequently resort to the use of VPNs to get around these bans, though there have been reports that the use of VPNs has also started to be blocked.
The government says such measures are always temporary and needed for national security but critics see them as another restriction on civil liberties under Erdogan.
In November 2016, Turkey imposed restrictions lasting several hours on the messaging service WhatsApp as well as on Twitter, Facebook, YouTube and other sites following the controversial arrests of pro-Kurdish MPs.
Prime Minister Binali Yildirim acknowledged at the time that "from time to time for security reasons we can use such measures.... These are temporary measures. Once the danger is passed, everything returns to normal."
Amid uproar on social media over the latest ban, there was also speculation the decision may also have been prompted by deeply unflattering updates to Erdogan's Wikipedia profile after he won the April 16 referendum on enhancing his powers.
The government insists that the new presidential system -- largely due to come into force in 2019 -- will improve efficiency, but critics fear it will lead to one-man rule.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)