The incident took place in Paris at a meeting where Hollande had given an address to a charity for the homeless.
As he was signing a pledge in support of the charity, a woman climbed onto the stage and made towards him with a bag of flour.
She managed to tip the contents over Hollande, covering him in the white powder, while shocked guards hurriedly wrestled her to the ground.
Hollande appeared unperturbed by the attack and even continued signing the contract.
Later he said that such attacks were "a risk of the job."
"There's always someone a bit more deranged than others," he told the media. "The main thing is that I was able to deliver the message I wanted to on the housing situation and was well received by this charity."
The woman was named by media as Claire Seguin from the northern town of Lille. She claims to have had housing difficulties and to be under attack from local Socialists.
A police officer from the elite protection squad SPHP (Service de Protection des Hautes Personnalités) told Le Parisien newspaper that bodyguards had failed to do their job.
"On the pictures you can see that his escort is too far away and badly positioned," he said.
"Candidates don't like a heavy police presence as it's damaging for their image. However, sometimes it's necessary to impose our methods."
"Yesterday it was flour, but it could have been acid," he said.
As reported in The Local on Wednesday, fears about the security of candidates have led to police protection being assigned to the Green party candidate Eva Joly.
The Norwegian-born former magistrate has received threatening messages since she launched her campaign, including a letter containing a bullet.
By coincidence, Hollande's own protection was already due to be increased from Thursday.