The number of unemployed in mainland France grew by over 50,000, or 1.4 percent, to 3.56 million, according to the labour ministry.
The rise was the biggest in a single month since January 2013 and follows on a slight decrease in July.
Hollande has previously said that he would only seek re-election if he succeeds in lowering the rate of unemployment.
A website in support of Hollande was supposed to have been launched on Monday evening, but was delayed as a result of the disappointing unemployment figures, according to reports.
The government said the jihadist attacks in Nice and Normandy in July, which led to a significant fall in tourism, were partly to blame.
Labour Minister Myriam El Khomri said the new figures were “sharply less favourable than those for previous months”.
She explained that some sectors had been particularly badly hit by the recent terror attacks.
The economy had been punctured in “areas linked to tourism, notably hotels, restaurants and the leisure industry,” the minister said.
The number of unemployed in all of France, including overseas territories, now stands at 3.81 million, the labour ministry said.
The figures are a further setback for the unpopular Hollande, who has said he will not stand for re-election April if he has not made a “credible” dent in unemployment.
Over the past year the number of unemployed has fallen by only 10,900. In the second quarter the unemployment rate was 9.6 percent.
Former president Nicolas Sarkozy, who is a candidate for next year's presidential election, said the government was following “an economic policy lacking good sense”.
But Mathieu Plane of economic think tank OFCE said the size of the unemployment rise was “worrying” and can't all be explained away by the attacks.
“It is also clear that growth has stalled, after a recovery phase up to the first quarter of 2016,” the analyst said.