Independent centrist Mr Macron came first in the first round and polls have shown him consistently ahead.
Read on for a look at Ms Le Pen’s life and policies and learn more about her rival in the run-off.
Marine Le Pen has spent much of her life under the shadow of her father Jean-Marie Le Pen, who founded the party she now leads, the National Front.
Born in 1968 in the leafy neighbourhood of Neuilly-Sur-Seine, just west of Paris, she survived a bomb attack on the family house when she was eight. A lawyer by training, Ms Le Pen showed an early interest in politics, accompanying her father to rallies from a young age and joining the National Front when she was 18. She was first elected to a regional post in 1998, marking the beginning of her political career.
As she rose the ranks of the party, becoming its leader in 2011, Ms Le Pen sought to purge it of its most overtly anti-Semitic and racist overtones – even if it meant fighting with her father. The pair fell out in 2015 when Mr Le Pen reiterated his stance that gas chambers were a “detail of history” – and was expelled from the party in a public row with his daughter.
The 48-year-old is twice divorced and has three children from her first marriage. She remains very private about her personal life.
Her party – Front National
The National Front was a fringe movement of xenophobes, nationalists and former Nazi collaborators when it was created in the early 1970s. In the 1980s it came out of political wilderness by forging some alliances with other right-wing movements – and won seats at the European elections in 1984.
Its biggest success to date came in 2002, when Mr Le Pen beat the Socialist candidate in the first round of French presidential election and went to a run-off with conservative Jacques Chirac. (Mr Le Pen finished a distant second.)
Today, the party keeps a tough stance against immigrants and is anti-EU, but under Ms Le Pen and her “de-demonisation” efforts it has softened its image. It now draws support from across the political spectrum by tapping into disgust over France’s 10% unemployment rate and political corruption scandals.
Riding the wave of populism sweeping across Europe and America, Ms Le Pen presents herself as the anti-establishment candidate and has welcomed both Brexit and the election of Donald Trump. She speaks out against what she calls the “two totalitarianisms” of the new millennium: globalisation and Islamic fundamentalism.
The “144 commitments” of her manifesto amount to a nationalist agenda. In a campaign pledge that echoes Mr Trump’s “America First”, she has vowed to put French people first. “There will be no other laws and values in France but French,” she said as she launched her campaign in Lyon on 5 February.
If elected, Ms Le Pen would pull France out of the eurozone and hold talks with European partners to reshape the EU into a loose alliance of nations. If this fails she would hold a referendum on EU membership (she calls the bloc “a failure”). She would also like to withdraw France from NATO’s integrated command.
Domestically, Ms Le Pen wants to lower the retirement age, slap taxes on imports and on the job contracts of foreigners, and reduce regular immigration to 10,000 people a year.
She had previously come out in support of the death penalty, but this has been dropped in the recent manifesto.
Did you know?
Ms Le Pen’s real name is Marion Anne but she was called Marine from an early age. As a young woman she had a reputation for partying.
She was furious at her mother, who posed for the French edition of Playboy in the 1980s with the intention of hurting her estranged husband Jean-Marie.
Though fiercely anti-EU, she is a former member of the European Parliament, and was ranked among the 100 most influential people by Time magazine in 2011 and 2015.
Ms Le Pen is running under the slogan “In the name of the people”, and her supporters at rallies are often heard chanting “On est chez nous” (“We are in our land”).