The 66-year-old former rebel leader won the July poll with 99.18 per cent of the vote, after eight other candidates including his most vocal critics were barred by the electoral commission.
Kagame won acclaim from Western and regional leaders for helping end the 1994 genocide and turning Rwanda into an attractive investment and aid destination. But his reputation has been sullied by allegations of rights abuses and suppression of dissent plus supporting rebels in Democratic Republic of Congo - all accusations he has denied.
Thousands gathered to witness Rwanda's President Paul Kagame be sworn in for a fourth term. (EPA PHOTO)
"For the last 30 years, our country has been good work in progress. This new mandate means the beginning of even more hard work," Kagame said.
"That expectation to keep improving is not a dream, it is a reality. We can do it and we will do it."
Sunday's swearing-in took place at Kigali's Amahoro national Stadium, with thousands in attendance, many wearing T-shirts in the yellow, green and blue colours of the national flag.
President Paul Kagame won acclaim from the West for helping end the 1994 genocide. (EPA PHOTO)
Kagame received a military 21-gun salute, accompanied by cheers from the crowd. Twenty-two heads of state from African countries were in attendance.
Rwanda's constitution was changed in 2015, allowing Kagame to extend his time in office.
The two men who vied against him last month - Frank Habineza from the Democratic Green Party and independent Philippe Mpayimana - both conceded defeat in the election, which rights groups say was marred by a crackdown on media, the opposition and civil society groups.
A government spokesperson repeatedly rejected such criticism during the election period.
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