The ban was ordered by the governmental Communications Service, said Asomiddin Atoyev, who heads the association of the country's internet service providers.
The orders came by email or text message, depending on the provider, Atoyev said.
Only one of the country's six main internet service providers and one of six mobile operators have chosen not to comply with the order, he said.
The Communications Service did not explain the reasons for the ban, and its leadership could not be reached for comment, Atoyev said.
BBC Russian said the ban may be backlash over a video report of the wedding of President Emomali Rakhmon's son Rustam. The video featured apparently inebriated Rakhmon Sr. singing and dancing.
YouTube was blocked in Tajikistan for more than two months between July and October last year and again for two weeks from March to April.
Independent media termed the first ban an attempt to stop reports about the government's largely unsuccessful military operation against a local warlord in the Gorno-Badakhshan province, while the second was linked to videos by opposition activists criticizing Emomali Rakhmon.
Tajikistan was ranked a "non-free" country in the 2013 Freedom in the World rating by US-based watchdog Freedom House.
About two million of the country's population of eight million are internet users, according to official figures from 2012.
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.