The government said the ban was to prevent students being distracted from studying during the exam period and to prevent the spread of false rumours.
The blocked sites include Facebook, Twitter, Instagram and Viber.
They will be unavailable for several days while university entrance exams are taken, a spokesman for the Ministry of Education said.
"It's a temporary measure until Wednesday. Social media have proven to be a distraction for students," said spokesman Getachew Reda.
Social media sites have gone down in Ethiopia before but only for a matter of hours, with the government previously denying any involvement. This is the first time social media sites have been publicly blocked nationwide.
"The government here is very keen to control social media," he said. "They will learn from this, next time there is a protest they will use the experience to do another nationwide clampdown."
Last week, the UN Human Rights Council passed a resolution classifying the censorship of the internet as a human rights violation.