In a letter of support, Redford condemned the looming auction in Paris and warned of "grave moral consequences" if it went ahead.
The masks -- described by French auction house Neret-Minet Tessier & Sarrou as kachina visages -- are due to go under the hammer on Friday.
Describing himself "as a close friend of the... Hopi culture," Redford wrote that the masks "belong to the Hopi and the Hopi alone."
"To auction these would be, in my opinion, a sacrilege - a criminal gesture that contains grave moral repercussions.
"I would hope that these sacred items can be returned to the Hopi tribe where they belong. They are not for auction," he added.
Representatives of the Hopi tribe, who number around 18,000, are appalled by the prospect of a slice of their cultural heritage being touted to the highest bidder.
The Hopi say the items being auctioned are blessed with divine spirits, and insist that even the mere description of them as masks or artefacts is highly offensive, adamant that the upcoming auction is a form of sacrilege.
But while the sale of sacred Indian artefacts has been outlawed in the United States since 1990 - legislation which has allowed the tribe to recover items held by American museums in the past - the law does not extend to sales overseas.
The auction house, however, has said there are no grounds to halt the sale, stressing that the items being sold were acquired legally by a French collector during a 30-year residence in the United States.