Polyana Viana dished out some justice of her own to a would-be thief. Photo / Instagram

A would-be thief has found out the hard way why it's a bad idea to try and rob a professional fighter.

UFC strawweight Polyana Viana was waiting for an uber outside her Rio de Janeiro apartment when a man threatened her and claimed to be armed.

"When he saw I saw him, he sat next to me," Viana told MMAJunkie. "He asked me the time, I said it, and I saw he wasn't going to leave.

"So I already moved to put my cell phone in my waist. And then he said, 'Give me the phone. Don't try to react, because I'm armed.' Then he put his hand over [a gun], but I realised it was too soft.

"He was really close to me. So I thought, 'If it's a gun, he won't have time to draw it.' So I stood up. 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.'"

UFC president Dana White shared a photo of the man who tried to steal Viana's phone on social media, calling it a "bad f***ing idea" – and it's fair to say it didn't turn out too well for the would-be thief.

The 26-year-old Viana has a 10-2 MMA record.

On the left is @Polyana_VianaDF, one of our @UFC fighters and on the right is the guy who tried to rob her #badfuckingidea

— danawhite (@danawhite) January 7, 2019