There's limited active opposition and little uncertainty about the result and two of the country's best-known opposition leaders remain in prison, while others are in exile.
Several opposition parties are boycotting the vote on Sunday, saying the process lacks credibility.
Polls are expected to open at 7am (5pm AEDT), with more than 3.2 million Congolese registered to vote, although analysts and civil society groups expect turnout to fall below the nearly 68 per cent recorded in 2021, when Sassou won with 88.4 per cent of the vote.
Sassou seized power in the oil-producing Central African nation in 1979 and has governed almost continuously since, except for a five-year hiatus in the 1990s.
He is running against six little-known candidates, none seen as serious challengers with electoral bodies dominated by figures aligned with the ruling Congolese Labour Party.
"The opposition is fragmented and lacks a strong, emblematic figure," said Remadji Hoinathy of the Pretoria-based Institute for Security Studies.
"There may be fatigue among voters, but that will not affect Sassou's chances."
Rights groups say the political space has narrowed in recent years, citing arrests of activists and suspensions of parties.
Prime Minister Anatole Collinet Makosso has dismissed accusations of pro-Sassou bias, arguing that Congo's oversight bodies will ensure a transparent and fair election.
Some Congolese say they see little chance for change.
"It's an election whose outcome is known in advance," said Frédéric Nkou, an unemployed resident of Brazzaville.
"I don't expect things to improve."
Sassou has campaigned on continuity, pledging to accelerate development projects and expand access to education and professional training.
The economy, heavily dependent on oil, has stabilised in recent years after a long downturn, but 52 per cent of Congo's 6.1 million people live in poverty, according to the World Bank.
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