The California Gurls singer is the latest celeb to join Cheryl, Louis and Simon at the X Factor 2010 auditions, and we're sure she was hoping for the luck of the Irish as she helped them search for the city's next big thing.
With Dannii away on maternity leave, a string of celeb judges have wowed the crowds, including Geri Halliwell and Natalie Imbruglia, but Katy's appearance certainly caused the biggest stir yet with fans, as she was greeted by an elated audience as she made her way inside to join the other judges.
Katy certainly held her own on the panel, and gave Cheryl a run for her money in the style stakes, opting for a sparkling figure-hugging mini by Jenny Packham and some rather tactile velvet fuschia heels, which she finished with loose voluminous locks.
Looking pretty as a picture, Katy promised to she would be a fair judge, but remarked that but if someone's bad, 'someone needs to tell them.' And was clearly feeling ambitious, hoping to 'find someone in Dublin who's going to change the world'.
We're betting she sat through more than a few dubious renditions of I Kissed A Girl in the process!
By Hayley Spencer