Le Pen told a press conference that she and Dupont-Aignan, who lost in the election's first round with 4.7 percent of the vote, shared a "common project that we will promote together".
"We will build a national unity government that will bring together people chosen for their skills and their love of France," Le Pen said.
During the bruising campaign, Le Pen has attempted to wear down lingering resistance to her National Front party's tainted brand by portraying her centrist rival Emmanuel Macron as an elitist money man.
Polls give him a commanding lead of up to 20 points over Le Pen in the runoff but show the gap narrowing slightly after Macron's sluggish start to his second-round campaigning.
Le Pen says she wants to build up France's borders, take it out of the eurozone and hold a referendum on the nation's EU membership.
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