Interior Minister Raymond Zephyrin Mboulou announced the results at 3:30am (0230 GMT) on national television, saying Sassou Nguesso had secured 60 percent of the vote in the tense weekend poll held under an ongoing communications blackout. Sassou Nguesso has ruled Congo for all but five years since 1979, having lived in Paris in exile from 1992 to 1997.
The official count gave runner-up Guy-Brice Parfait Kolelas 15 percent of the vote, while General Jean-Marie Michel Mokoko came in third with 14 percent.
Sassou Nguesso hailed the victory at his campaign headquarters, saying the Congolese people had "taken their destiny into their own hands" and adding that the campaign had produced a "very open" democratic debate in the former French colony.
Both of his main rivals had already rejected the partial results released Wednesday, with Kolelas' spokesman Vivien Manangou saying there had been "massive fraud".
With telephones and the Internet cut off, neither candidate was immediately reachable after the official results were announced.
Critics accuse him of rampant corruption and nepotism, blasting the referendum result as a "constitutional coup".
Authorities imposed a communications blackout during the election to prevent opposition candidates from publishing "illegal results". A government source said they blackout would remain in place until after the official results.
The European Union refused to send election observers to monitor the polls, saying conditions had not been met for a transparent and democratic vote.