The 92-year-old has been the president of the Central African coastal state since 1982. He is standing against 10 men and one woman.
Only the dictator in Cameroon's neighbouring country Equatorial Guinea, Teodoro Obiang Nguema Mbasogo, has been in office longer worldwide.
Cameroon is a multi-party democracy but opposition, civil society and freedom of opinion and press are severely restricted.
Of Cameroon's approximately 30 million inhabitants, eight million are registered voters, representing only slightly more than half of people of voting age.
The results are expected to be announced two weeks after the election. The winner will be the candidate who receives the most votes.
Biya received 71 per cent of the votes in 2018, according to official results, while the runner-up received 14 per cent.
This year, some of the strongest contenders are former minister and government spokesman Issa Tchiroma Bakary, 76, and ex-premier Bello Bouba Maigari, 77, who left the government in the summer to run against Biya. Both are from the vote-rich north of the country.
Cameroon, once a German colony, was split between France and Britain after World War I and gained independence in 1960.
In a conflict between the French-speaking majority and the English-speaking regions of the country, at least 6500 people have been killed and around 600,000 displaced since 2017.
The north is further roiled by attacks by the Islamist terrorist group Boko Haram from Nigeria.
Cameroon's further woes include debt, corruption, poor infrastructure and youth unemployment.
Some 40 per cent of the country's inhabitants are estimated to live in poverty.
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