The president has ruled the country for 42 years - and will now continue for another five.
The announcement in the Central African country was broadcast on television on Tuesday. Voter turnout was reported at just over 84 percent.
In connection with the election, internet traffic was shut down, vehicle traffic was halted in the oil- and gas-rich country, and shops were urged to remain closed.
Six opposing candidates challenged Sassou Nguesso, but the opposition was divided and criticized the election for being rigged.
Sassou Nguesso is one of the longest-serving leaders in Africa, along with Equatorial Guinea's Teodoro Obiang Nguema Mbasogo and Cameroon's President Paul Biya.
