Web monitoring companies reported regular traffic on the internet plummeted to zero just before 19:00 GMT on Tuesday (22:00 local time).
Syria last experienced a shutdown for three days in November 2012.
The government blamed that incident on “terrorists”, but internet experts said it was more likely that the government had deliberately shut down the web.
The regime of President Bashar al-Assad has been fighting a bloody internal conflict for two years.
Activists suggested at the time of the last internet shutdown that the government might have been planning a major offensive, or that it might have been attempting to disrupt rebel communications.
However, neither theory was substantiated.
US web companies including Renesys and Akamai logged the latest blackout late on Tuesday.
Jim Cowie of Renesys told the BBC there was not enough information to tell what had caused the latest blackout.
“It seems to be very similar in terms of the dynamic to the event that took place last November in that the routes to the Syrian internet were withdrawn very suddenly and all together, almost like a switch being thrown,” he said.
“One can always speculate about whether it is infrastructure damage, or power outage at a critical facility or simply someone deciding to turn off the internet.”
Syrian residents confirmed the blackout, but said mobile phones and landlines were working normally.