In a breaking news alert, Syrian state television announced the Internet and communications were back up and running.
Landline phone services between Syrian provinces had also been down since Tuesday, state news agency SANA said.
US tech firms and the US State Department reported the blackout on Tuesday but did not specify any reasons for it. A similar blackout happened last November.
"Internet services and phone calls between provinces were cut off Tuesday evening because of a fault in optical fibre cables," SANA said before service was restored.
"Efforts are ongoing to fix the faults and to bring Internet and telephone services back up as soon as possible," the agency said, quoting a communications official.
Activists who frequently use the Internet to report on violence engulfing their country blamed the authorities for the blackout.
"Even satellite communication devices" used by many anti-regime activists to avoid surveillance "have been slow," a Syrian activist currently out of his country told AFP's Beirut bureau.
"I think the regime has a plan to stage some kind of attack. That's what happens every time Internet goes down," said the activist, an Internet expert who identified himself as Fares.
The Syrian Observatory for Human Rights said the blackout appeared to be a deliberate act to help regime forces carrying out military operations.
The United States also voiced concern.
"We condemn any effort by any group to restrict or eliminate the Syrian people's access to information and communications of any kinds," Patrick Ventrell, the deputy State Department spokesman, said in Washington.
"These shutdowns are hard to attribute to one side or the other, and technical groups are analyzing them. But the regime has a history of restricting the Internet in a range of ways to prevent the Syrian people from accessing and sharing information."
Syria is ranked 176 out of 179 countries in a worldwide press freedom index compiled by international press freedom watchdog Reporters Without Borders (RSF).