Although it was initially her form-fitting pastel green dress that caught everyone's eye, it was the dazzling rock on her left hand that really got people talking.
The Roar singer did little to dismiss the rumours that singer John Mayer might have proposed to her.
When asked about the sparkler she said, "Oh, this little old thing. I' m just having fun and dressing up in my Zac Posen dress and vintage headpiece from Tokyo."
The 29-year-old complemented the simple diamond band with statement diamond stud earrings at MTV's European Music Awards in Amsterdam.
Completing the dazzling look was her multicoloured head piece.
The couple have had an on-off romance since they began dating in the summer of 2012.
They are thought to have split up in March of this year but were back together by the summer.
John opened up about his 'human' romance with Katy shortly after they were first linked.
"I don't feel like I'm in a celebrity relationship," said the singer, who has dated Jennifer Aniston, Jessica Simpson and Taylor Swift in the past. "I don't feel it for the first time in my life.
"I'm not even in a high-profile relationship…I know it's high-profile, but it's hard to explain. It doesn't feel that way to me, and it's great and I'm glad. I'm saying for me, it feels like it's something very human."
Katy has also spoken about her feelings for John.
"I actually am dating my crush," she told W Magazine. "I had a crush on him for a long time."
If the couple are engaged it will be the first marriage for John. Katy was married to comedian Russell Brand for 14 months before they split in December 2011.