"I share the emotion, I share the compassions, I share also the outrage," the French leader said upon arrival at the French Indian Ocean island of Reunion.
"To kill, to assassinate a journalist in these conditions, a journalist who had already been held for two years, to put on the internet the video of his murder, that is a sign of barbarism."
Hollande added: "there is a rule which applies in all civilised society that the press must be protected."
The Islamic State group who killed Foley and who have declared the creation of a caliphate straddling Syria and Iraq are not like other terrorist groups, the French leader warned.
"Dispersed, scattered, with several heads, it is a terrorist organization that has decided to enslave, to annihilate, to destroy," he said.
"And if the world doesn't organise regarding this group there will be other equally appalling images, which won't only concern journalists, they've crucified people," he added.
Hollande repeated a call for an international conference on tackling the Islamic State and on protecting Iraq's security.
Condolences and shocked messages poured in worldwide after the Islamist group released a video of Foley's beheading late Tuesday.
Foley, 40, had been missing since he was seized in Syria in November 2012.