"The day has come, and this truth must be told," Hollande said in the first presidential visit to the main internment camp for Roma, located in Montreuil-Bellay, central France.
"The (French) Republic acknowledges the suffering of travelling people who were interned and admits that it bears broad responsibility," Hollande said.
Roma, also known as gypsies, were brutally persecuted in the Holocaust, parallelling the systematic murder of Jews. Estimates of how many died vary widely, between 220,000 and half a million.
The Vichy regime is the term for the government set up in France – but under de-facto Nazi control -- after France surrendered to Germany in 1940.
The Vichy regime fell in late 1944 when the allied forces reconquered France and General Charles de Gaulle set up a provisional government.
Between 6,000 and 6,500 Roma were interned in 31 camps, the biggest of which was Montreuil-Bellay, where more than 2,000 were confined between November 1941 and January 1945. About a hundred of them died.
The camp was also used to intern a number of people from the city of Nantes who were officially categorised as homeless.
Some Roma remained interned in French camps until 1946.
"Nearly all families of travelling people have at least one relative who passed through Montreuil-Bellay," Hollande said.
More than 500 people took part in Saturday's ceremonies, held 66 years after the last interned Roma had been set free, including some survivors as well descendants of the victims.
"It was important to us to have this recognition. It affects thousands and thousands" of Roma families, said Fernande Delage, head of the France Liberte Voyage NGO.
"It's late, but better late than never," he added.
Lucien Violet, a 69-year-old whose parents were held in Montreuil-Bellay, also attended the ceremony.
"This is the first president to pay homage to travelling people. We feel genuinely moved by his presence," Violet said.
"Our families have suffered enormously and we will never forget, even though there is forgiveness," he added.
At the site, a commemorative art installation by ceramics artist Armelle Benoit was set up, comprising a portico of eight columns engraved with the family names of the 473 affected families.
Hollande on Saturday also threw his weight behind moves in parliament to scrap a 1969 law that defenders of minorities say is discriminatory.
The legislation traces its roots to a regulation in 1912, which aimed at pressing Roma to settle down. It required "nomads" to have a special ID card.
This was replaced in 1969 by the requirement for "travelling people" to have a specific set of papers and name a district as their home base.