Ruling party candidate Mohamed Bazoum is seen by many as the favourite after leading in the first round on December 27 with 39.3 percent of the vote. Polling stations opened at 0700 GMT and will close at 1800 GMT (8 am to 7pm) local time.
Voters in Niger are casting ballots in the country's second round of a presidential election that is expected to usher in the first democratic transition of power since the West African nation gained independence from France in 1960.
Nigeriens will go to the polls on Sunday after none of the 28 candidates won a majority during December's vote.
Ruling party candidate Mohamed Bazoum is seen by many as the favourite after leading in the first round on December 27 with 39.3 percent of the vote. He is up against former President Mahamane Ousmane, who scored 17 percent.
Niger is the world's poorest nation, according to the UN's benchmark of development of 189 countries, and is struggling with militant insurgencies that have spilled over from Mali and Nigeria.
Only 7.4 million of the country's 22 million population are eligible to vote on Sunday – the rest are under-age.
Thousands of soldiers were deployed across the country for the vote, which is on track to usher in a peaceful handover between elected presidents.
Outgoing President Mahamadou Issoufou's decision to voluntarily step down after two five-year terms was welcomed in a region where many leaders have tried to cling on to power.
His successor will either be his right-hand man and anointed successor Mohamed Bazoum or Mahamane Ousmane, who became the country's first democratically- elected president in 1993, only to be toppled in a coup three years later.
Ousmane, 71, is running for president for the fifth time since his ouster.
He has been endorsed by around a dozen smaller parties and candidates of the first round. He has promised to bring change and tackle corruption.
Both candidates are stalwarts of Niger's political scene.
Winning votes alone has not always led to power in Niger – there have been four military coups and six elections since independence.
But political alliances will loom large on Sunday, when Bazoum will be the strong favourite after winning 39.3 percent of the first round vote on December 27.
The 60-year-old former interior minister has already sealed the support of the candidates who came third and fourth in the first round.
"Bazoum has a coalition that is expected to win if the voting instructions of the parties which support him are respected by their activists," said Ibrahim Yahya Ibrahim of the International Crisis Group (ICG) thinktank.
But, he cautioned, it was "very far from being won".
Ousmane took nearly 17 percent in the first round.
He can count on the support of a coalition of 18 opposition parties called Cap 20-21 as well as Hama Amadou, who had previously been thought to be the most formidable candidate against Bazoum.
Amadou was ruled out from running due to a prison sentence for baby trafficking which he has slammed as politically motivated.
The opposition filed an unsuccessful appeal against the result of the first round, claiming fraud.
A diplomatic source, speaking on condition of anonymity, said the concern was not so much "unfair practices" but the ruling party's imposing electoral machine.
The elections "aren't perfect, at least they are elections", the source said.
Bazoum has campaigned on continuity with the previous regime, which has promised development in a country struggling with the highest birthrate in the world — an average of seven children per woman.
"To absorb this population growth would take incredible economic growth," a Western diplomatic source said.
An immense security challenge also awaits whoever emerges as victor.
A brutal insurgency is intensifying in the country's west, while Islamist militants from the Nigerian movement Boko Haram wage incessant attacks in the southeast.
Like its Sahel neighbours Mali and Burkina Faso, Niger is increasingly struggling to cope.
Hundreds of troops have been killed and hundreds of thousands of people have fled their homes.
Polling stations opened at 0700 GMT and will close at 1800 GMT (8 am to 7pm) local time.