The country's electoral commission announced Mr Kabila won 58.05% of the vote, ahead of ex-rebel leader Jean-Pierre Bemba who got 41.9%.
A member of Mr Bemba's camp has vowed to challenge the vote by all means.
International peacekeepers have deployed extra troops in the capital, Kinshasa - a Bemba stronghold.
Mr Kabila told the BBC the country should remain quiet because a new page of its history had just been turned.
But the BBC's Mark Doyle says Mr Kabila, who obtained relatively few votes in the capital and in the north of the country, will face major difficulties ruling Congo.
Kinshasa is quiet and the streets empty, says the BBC correspondent there, Arnaud Zajtman.
But in Lubumbashi, DR Congo's second town and a stronghold of Joseph Kabila, people are celebrating.
Final results were not expected until Sunday, but the electoral commission announced the official verdict on Wednesday evening, despite objections already lodged by Mr Bemba's team.
The result must now be upheld by the supreme court.
Poll officials have rejected claims of fraud from Mr Bemba's camp.
Electoral commission head Apollinaire Malu Malu called for candidates to respect election rules after the Bemba coalition said their candidate had received more than 50% of the vote and that victory "was being stolen from the Congolese people".
An analyst who wished to remain anonymous told the BBC that there were serious questions about the validity of some ballot papers, especially a large number of votes cast by voters outside their home areas.
Our correspondent adds that there were big question marks over Mr Kabila's tactics ahead of polling, when soldiers intimidated voters and he used the national TV station as a propaganda tool.
The Carter Center mission said they believed fraud was virtually impossible after ballots had been counted at each polling station.
At least four people were killed in Saturday's clashes
Mr Bemba has not commented on the results, but a member of the coalition backing the politician vowed to challenge the vote by all means.
"The Union for the Nation will not accept an electoral hold-up that aims to steal victory from the Congolese people," the coalition said in a statement.
The vote has been the first following DR Congo's five-year conflict.
Forces loyal to the two candidates clashed during the war as well as during the tense election period.
Following violence on Saturday in which four people died, the police arrested 337 homeless people, including 87 children, the government says, blaming them for starting the trouble.
Eyewitnesses say that security forces loyal to the two candidates exchanged gun and mortar-fire.
United Nations observers say the election is the most significant in Africa since Nelson Mandela was elected as South Africa's president in 1994.
The first round of elections showed a regional divide, with Mr Kabila gaining a landslide in the Swahili-speaking east, while Mr Bemba got most support in the west, where Lingala is the common language.
The world's largest peacekeeping force - 17,000-strong - is in DR Congo, tasked with ensuring security.
At least 23 people were killed in gun battles between security forces loyal to the two men in Kinshasa after the announcement of first round results.
Mr Kabila won 45% of the vote, while Mr Bemba got 20%.
International observers generally praised the vote as being well-run, despite some disruptions in the north-east of the country.
Source : BBCGüncelleme Tarihi: 20 Eylül 2018, 18:16