Viana said she thought the man was possibly armed with a toy gun or a knife.
"He was really close to me. So I thought, 'If it's a gun, he won't have time to draw it,'" Viana told the news site.
Viana stood up. It was then that the man learned that the red MMA T-shirt the 5-foot, 5-inch, 115-pound woman was wearing did not mean she was a fan of the sport.
"I threw two punches and a kick. He fell, then I caught him in a rear-naked choke. Then, I sat him down in the same place we were before and said, 'Now, we'll wait for the police,'" she said.
While waiting, Viana said, she kept the man’s arm immobilized. She also pulled out her phone and took at least one photo of the bloodied, swollen-faced attempted robber.
Dana White, president of the UFC, shared the photo on Instagram, alongside a photo of Viana.
“On the left is @polyanaviana, one of our @UFC fighters, and on the right is the guy who tried to rob her. #bad(expletive)idea,” White wrote.
The photo has since gone viral, the incident making news across the globe. Viana received praise for the way she defended herself.
Several people pointed out that she beat the man up while wearing flip flops. Others shared a photo of the cardboard weapon the man is accused of using in the attempted robbery.
Viana told MMA Junkie that the man initially tried to play off his actions.
"He told me to let him go, like, 'I just asked for the time,'" Viana told the website. "I said, 'Asked for the time my (expletive),' because he saw I was very angry. I said I wouldn't let go and that I was going to call the police.
“He said, ‘Call the police, then,’ because he was scared I was going to beat him up more.”
The MMA fighter said she later learned she had been correct -- the man was armed only with a piece of cardboard he had cut into the shape of a handgun.
Rio de Janeiro police officials have not identified those involved in the incident but confirmed to CNN Sport that an attempted robbery had taken place.
"A criminal was found immobilized by civilians after trying to rob a person on site," a police statement read. "A fake gun was seized with him."
Viana told CNN Sport that she remained calm throughout the incident, partly because she sensed the man was a thief when he sat next to her.
"I did not do it on impulse," Viana told the news network. "I was calm. I knew what I was going to do, but I only did it because I knew he did not have a gun."
She urged other robbery victims to not use her actions as inspiration in similar situations.
"I would not recommend it at all. I'm a trained fighter," Viana said, according to CNN. "I knew how to defend myself because I have knowledge and more so because I managed to stay calm."