Mélenchon is a former Socialist lawmaker who gained 11.1 per cent of the votes on the first round of the last presidential election in 2012.
At the time the country's Communist party backed Mélenchon's candidacy. Their Left Front union had not fared well in local elections since and this time, Mélenchon has not struck an alliance with the party.
Without the party's backing, it is not clear how he will finance his campaign. But at least one recent opinion poll has suggested as much as 10 per cent of left-wing voters consider Mélenchon the best candidate for the Left in 2017, only three percentage points behind Hollande.
"I am presenting my candidacy for 2017," Mélenchon said on a video posted on his website. "This election can be a chance for the people. It is the opportunity to turn the page peacefully and democratically on an unfair and cruel system our country and continent are sinking into."
Hollande is trying to discourage candidacies to his left, arguing that they increase the risk that no left-wing candidate will make it to a runoff between the top two finishers on the first round of the 2017 election.
The French president is expected to announce a government reshuffle later this Thursday, in a bid to reshape his team ahead of the 2017 presidential elections.