The agreement has caused immense discord in the Socialist party, which for years had dominated French politics.
France's left-wing parties have formed a historic agreement to create a new union ahead of the June legislative elections.
The agreement was confirmed overnight by a Socialist Party internal vote in favour of joining the far-left party La France Insoumise (LFI) along with the Green Party and the Communist Party in a coalition.
The four left-wing parties will campaign together in a New People's Ecological and Social Union and will not run candidates against each other in the elections for the lower house of parliament on 12 and 19 June.
They aim to win a majority of seats in an effort to counter President Emmanuel Macron.
"The result of the legislative elections is very uncertain," said Paul Bacot, an emeritus professor at Sciences Po Lyon.
"Will Insoumis (far-left) voters vote for socialist candidates, and vice versa? Will the dynamic of the union mobilise additional voters, or will the absence of candidates from the various components of the left lead to massive abstention?"
Bacot said the results of the presidential election may end up being very different for the legislative parties.
The negotiations between the parties were driven by LFI's candidate, Jean-Luc Mélenchon, who came third in the first round of the presidential elections on April 10, just shy of far-right candidate Marine Le Pen who faced Macron in the second round.
The other left-wing parties received under 5% of the vote, with the formerly dominant Socialist Party obtaining a historic low of under 2% of the vote.
While Macron was re-elected with 58% of the vote, many left-wing voters told Euronews they were disappointed with the president's policies and faced a difficult choice between voting or abstaining in the second round.
LFI and the Socialists said in their agreement on Wednesday that was later approved by the party that they "share common programmatic goals that will form the basis of a shared government programme of several hundred proposals."
But the vote has caused discord among the Socialists, particularly on the topic of Europe.
The pro-EU integration party agreed with the more eurosceptic LFI to "put an end to the liberal and productivist course of the European Union and to build a new project at the service of the environment and solidarity."
The agreement prompted former Prime Minister Bernard Cazeneuve to quit the party. Lille's Socialist mayor, Martine Aubry, meanwhile urged support for the agreement despite her "major reservations" about the stance on Europe.
"Left-wing voters had expressed in the first round of the presidential elections great hope for togetherness and unity. We must listen to this message," said Aubry.
For Bacot, there were only "bad solutions" for the Socialists after their historically low score in the presidential elections.
"The alliance with (far-left) LFI guarantees it in principle a number of candidates, constituencies and elected officials allowing it to maintain public funding," said Bacot.
But it comes at the cost of "heavy compromises" and possible "numerous defections" from the party.
The president's party has already aimed to welcome members of the Socialists who are unhappy with the agreement.
Health minister Olivier Véran, who quit the Socialist Party in 2016 to join Macron, told disappointed party members that the party had left them and urged them to join the president.