The 28-year-old singer's number was originally scheduled for a September 8 release, despite being available in most other countries, reported Digital Spy.
"Attention UK! I'm letting 'Roar' out a week early. Go here to pre-order now, and you get the song this Sunday, September 1," she posted on Twitter.
Perry recently revealed that her new album Prism won't contain any "darkness" and it will be released worldwide on October 22.
She will also close this year's iTunes Festival by headlining the month-long event on September 30.