It was not initially clear why the ban had been imposed.
The Turkey Blocks group said the site was inaccessible from 08:00 by order of the Turkish authorities.
People in the capital Istanbul were unable to access any Wikipedia pages without using a Virtual Private Network.
“After technical analysis and legal consideration based on the Law Nr. 5651, an administrative measure has been taken for this website,” Turkey’s Information and Communication Technologies Authority was quoted as saying.
Turkey Blocks and Turkish media, including the Hurriyet Daily News, said the provisional order would need to be backed by a full court ruling in the next few days.
Social media was in uproar as news of the ban emerged, with some users speculating that it might be a bid to suppress criticism on President Recep Tayyip Erdogan’s Wikipedia page.
Mr. Erdogan narrowly won a controversial April referendum on increasing his powers, but the issue has deeply divided the country.
Turkey has temporarily blocked popular social media sites including Facebook and Twitter in the past, especially in the wake of mass protests or terror attacks.
The government has previously denied censoring the internet, blaming outages on spikes in usage after major events.