Assad swore by the Koran before the country's parliamentarians in an extraordinary session, 40 months into a conflict estimated to have killed more than 170,000 people.
"Syrians, three years and four months... Have passed since some cried 'freedom'," he said, referring to the start of a March 2011 revolt that demanded his ouster.
"They wanted a revolution, but you have been the real revolutionaries. I congratulate you for your revolution and for your victory," Assad told his supporters.
More than 1,000 people were invited to the inauguration ceremony, with Assad arriving at the presidential palace in a black sedan car before being met on a red carpet by a military band.
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Parliamentarians and other guests cheered for Assad in the hall where Assad was sworn in and then gave a speech.
Assad won a June 3 election held only in regime-controlled territories by 88.7 percent, defeating two other candidates seen as figureheads, rather than genuine opponents.