"Today is a historic day. We have taken a decision to ally ourselves with other patriots willing to work within the same dynamic," Le Pen told journalists after seeing Wilders.
"Today is the start of the liberation of Europe from the monster of Brussels," the Dutch far-right chief said.
In order to form a far-right anti-European bloc, Wilders and Le Pen would have to find like-minded politicians in at least a quarter of the EU's 28 member states and see 25 members elected to the 766-seat European Parliament.
If they manage to form an official European political group they will benefit from subsidies, offices, a communication budget, seats on committees and speaking time in parliament proportional to their number.
There is already a eurosceptic parliamentary group called the Europe of Freedom and Democracy (EFD), led by Nigel Farage of Britain's UK Independence Party but it only has 32 seats.
Farage is reported to be wary of allying with Wilders and Le Pen because of their far-right image.
"I understand that he is not too eager today to work with my party, but let me tell you, I hope after next year's elections he will be able to join in our initiative," Wilders said.
A Le Pen-Wilders alliance could include the Alternative for Germany (AfD), Belgium's Vlaams Belang, Italy's Northern League, the Freedom Party of Austria and others.
But right-wing parties in eastern Europe, including Hungary's Jobbik and Slovakian, Bulgarian and Romanian nationalist because they are judged "too racist".
There have already been similar initiatives on the European far-right, says Magali Balent, a specialist of the far right at the Robert Schuman Fundation in Paris.
"The difference today is that these parties are being pragmatic because they realise they are popular," she told RFI. "They also want to come to power and for that they need to be credible on the international stage. First in Europe but also beyond. They want to show that they are parties that can get organised, work together and that have views that matter outside their own country."
If the grouping is a strategic question, it is also has an ideological basis, says Balent. "It’s also an ideological issue because both parties have a number of ideas in common."
Although she faces legal action for comparing Muslim prayers in the street to the German occupation of France, Le Pen has said that Wilders is "perhaps more radical" in his opposition to Islam than her.
For his part, Wilders, a strong supporter of Israel, criticised anti-Semitic remarks by Le Pen's father Jean-Marie, but on Wednesday he said he was "convinced that there's not an inch of racism or anti-Semitism within her".
The alliance aims to unite eurosceptic parties amid rising discontent over economic crisis in the European Union and Brussels' reaction to it.
"Working together, we want to repatriate the ability to decide ourselves how we control our borders, how we control our money and our economy," Wilders said.