Currently, the top rate of income tax is 45% so a 100% rate would effectively cap earnings at €400,000.
"I believe that there is a limit to the accumulation [of wealth]. If there are any who want to go abroad, well, goodbye!” said Melenchon last month.
He also plans to increase the tax free allowance for inheritance to €130,000, up from €100,000, but hike the tax on amounts above this threshold.
France already has some of the world's highest rates of income tax, as well as an annual wealth tax of 0.5% starting at €900,000, increasingly gradually to a top rate of 1.5% to anything over €10m.
In recent years, this has led to an exodus of the country’s wealthy.
According to figures from research company New World Wealth, around 10,000 millionaires left France in 2015, followed by 12,000 last year.
This includes key figures such as actor Gerard Depardieu and the chief executive of LVMH, Bernard Arnault.
Previous attempts to increase the top rate tax, such as president Hollande’s proposal of a 75% rate in 2012, have quickly been rejected by the courts.
However, there are concerns that Melenchon’s rising popularity may lead to another wave of wealthy individuals leaving France.
He also wants to limit executive pay to 20 times that of the lowest-paid employee, ban companies from paying dividends if they’ve laid off workers for economic reasons, impose capital controls to fight tax fraud and expand the base of those paying the existing wealth tax.
The 65-year-old candidate could be one of two contenders to come out on top in the first round of the French elections on 23 April amid a tight head-to-head with far-right rival Marie Le Pen and centrist Emmanuel Macron.
If he wins, it will qualify him for the May 7 runoff, giving him a fair shot at the presidency.
Taxing the wealthy has been a contentious issue during the election campaign, with centre right candidate François Fillon calling for its abolition.
Macron wants it to cover only property, excluding investments that he says would allow more savings to be channelled into the real economy.
Meanwhile, on Wednesday, the UK parliament voted to approve British prime minister Theresa May’s request for a snap election on 8 June.
Her opposition, the Labour Party, hinted that anyone earning £70,000 or more would be classed as “rich”, sparking claims the party is planning to bring the 45% top rate of tax down from its current threshold of £150,000 to £70,000.