The pair topped the polls in the first round of voting on April 23.
- Why is the election important? -
France is the eurozone's second-biggest economy as well as a global military and diplomatic heavyweight, with veto power as a permanent member of the UN Security Council.
The second round will come down to a battle between the pro-European, pro-globalisation vision of Macron and Le Pen's hostility to the EU and NATO.
If Le Pen wins, it will further shake up the West's liberal post-war order, already rattled by Britain's vote to leave the EU and Donald Trump's election to the White House.
- How is the president elected? -
The president is elected directly in a vote of one or two rounds. If neither candidate obtains an absolute majority in the first round, a run-off is held two weeks later. Every presidential election since 1965 -- the first presidential election by universal suffrage since World War II -- has gone to a second round.
- Who ran in the first round? -
Eleven candidates spanning the spectrum from Trotskyist left to far-right took part in Sunday's first-round vote.
The main candidates were Macron, 39, Le Pen, 48, conservative Francois Fillon, 63, and hard-left firebrand Jean-Luc Melenchon, 65.
Macron finished first on 24 percent, ahead of Le Pen with 21.3 percent, Fillon with 20 percent and Melenchon with 19.6 percent.
- How does the election work? -
A total of 46.9 million voters are registered to vote in the election. Turnout in the first round was 77.8 percent.
The election is the first in the history of France's 59-year-old Fifth Republic to take place under a state of emergency, which was imposed after the November 2015 attacks in Paris.
More than 50,000 police backed by 7,000 soldiers from the Sentinelle anti-terror operation were on patrol during the first round.
- What happens next? -
The winner of the election will be sworn in by May 14 at the latest, taking over from Francois Hollande.
Hollande decided against seeking re-election after a single term marked by record low approval ratings.
A two-round parliamentary election will be held on June 11 and 18.
Analysts say both Le Pen's National Front and Macron's year-old En Marche (On The Move) movement would struggle to form a parliamentary majority.
This means that whoever wins the presidency will likely have to form a pact with members of the Socialist party, the conservative Republicans or both.