The elections "have proven the strength of popular legitimacy... conferred on the state," Assad said in his inauguration speech.
On the eve of the 26 May election, the United States, Britain, France, Germany and Italy said the poll was "neither free nor fair", and Syria's fragmented opposition has called it a "farce".
Syria's war has displaced millions of people since starting in 2011 with the repression of anti-government protests.
Return to embrace
Assad called on "those who bet on the demise of the homeland" to return to its "embrace".
With his campaign slogan, "Hope through work", Assad had cast himself as the sole viable architect of reconstruction.
In his speech, he noted that after over ten years of conflict, when security dominated the agenda, the focus now was on "liberating" areas still beyond Damascus' control and tackling the war's effects on the economy and people's livelihoods.
After a series of victories against jihadists and rebels, Russian-backed government forces today control two-thirds of Syria.
Syria's former Al-Qaeda affiliate runs the opposition bastion of Idlib in the northwest, where Turkish-backed rebels are also present.
A Turkish-Russian ceasefire has largely held in Idlib since March 2020, after halting a deadly government offensive that displaced a million people from their homes.
But violations of that truce have increased in the south of the bastion in recent weeks, the Britain-based Observatory says.
Kurdish-led forces control much of the oil-rich east after expelling the Islamic State jihadist group from the region with US backing.
And Turkey and its Syrian proxies hold a long strip of territory along the northern border.
Assad pledged to wrest remaining Syrian territory from "the terrorists and from their Turkish and American sponsors".
Assad takes his oath as the country faces a dire economic crisis.
More than 80 per cent of the population live in poverty, and the Syrian pound has plunged in value against the dollar, causing skyrocketing inflation.
In recent weeks, the government has hiked the price of petrol, bread, sugar and rice, while power cuts can last up to 20 hours a day amid fuel shortages.
Nationwide, 12.4 million people struggle to find enough food each day, the World Food Programme says.
The Damascus government has blamed the country's economic woes on Western sanctions and a deepening crisis in neighbouring Lebanon.
Banks in Lebanon have for more than a year forbidden depositors from withdrawing their dollar savings, affecting Syrian clients.
"The biggest obstacle now is the Syrian funds frozen in Lebanese banks," said Assad, estimating them to amount to tens of billions of dollars.
He spoke as Chinese foreign minister Wang Yi landed in Damascus, Syrian state media said, in the first visit by a Chinese government official since a deputy foreign minister in early 2012.
Assad was first elected by referendum in 2000 following the death of his father Hafez al-Assad, who had ruled Syria for 30 years.