Source: BBC
He secured the win with 63% of the vote, the electoral body Ceni said.
However, turnout was low amid an opposition boycott – just 16% of people voted in the presidential election.
Opposition candidates alleged instances of ballot stuffing in favour of Mr Assoumani and that polls had closed early, the Reuters news agency reports.
Mr Assoumani’s campaign team has denied the opposition’s claims.
Six candidates ran in the 14 January presidential election in the Indian Ocean archipelago.
“We cannot talk about results because there was no election,” Mouigni Baraka Said Soilihi, one of Mr Assoumani’s opponents, was quoted as saying by Reuters.
International poll observers have said that voting was largely free and fair despite the irregularities reported by the opposition.
According to the election commission, 189,000 voters went to the polls, but only 55,259 – just 16% of registered voters – took part in the presidential election.
The dismal turnout followed calls for a boycott by some opposition candidates, prompting some voters to only participate in the gubernatorial elections held at the same time.
More than 330,000 people were registered to vote, out of a population the World Bank estimates at 836,000.
Mr Assoumani is a former military officer who first came to power through a coup in 1999 and won his first election in 2002.
He exited politics in 2006, before making a comeback with a presidential win in 2016.
He was able to seek a fourth term after a controversial referendum removed presidential term limits in 2018, leading to widespread protests in the country.
Mr Assoumani’s rule has been marred by controversy, with critics accusing him of jailing and exiling his opponents.
He is the current chairperson of the African Union.