"I will say this out loud: I will not wear fur," Mori told AFP. "I love animals and I agree with those who oppose wearing fur products."
People for the Ethical Treatment of Animals (PETA) launched a campaign last week to press Mori to renounce fur, angered that she went on the catwalk with a full-length fur coat.
Mori and other contestants wore the fur coats during the final round of the preliminary pageant in Japan to select the national candidate, the Miss Universe Organization said.
"At that time I wasn't expecting to face this issue so soon as Miss Universe," Mori said.
But Mori said she felt the need to speak out about after being crowned Miss Universe in Mexico City earlier this year.
"I am invited to Fendi's show in China next week and I was encouraged to pick a coat because it will be very cold when I travel there," she said, referring to the Italian fashion house.
"The entire Fendi collection of winter coats which was offered to me to pick from was fur, but I said I would not wear one made with fur," she said.
"I will wear a white jumper that will be lent by Fendi."
PETA is famous for its publicity stunts and for targeting celebrities who wear fur and companies that sell it.
The US-based rights group wrote Mori a letter asking her to "prove that your beauty is more than skin deep" and encouraged its members to send her emails.
Ines Ligron, Miss Universe Japan national director who helped train Japanese contestants, said she encouraged Mori and other candidates to wear the fur coats in the preliminary contest.
Supporting Mori's decision to decline a Fendi fur coat, Ligron said: "She refused even though she was kind of pressured to wear fur."
Mori, 20, has said she wants to use her tenure as Miss Universe to fight HIV and AIDS and that she hopes one day to perform on Broadway.
She added she also wants to draw the attention of pet lovers in Japan, some of whom go to extremes to put name-brand outfits on their dogs and cats.
"I wonder how many children in Africa would starve to death while some people here spend a lot of money on their pets," she said. "We have to think about more than just what's right in front of us."