The singer's latest cut 'Part of Me' has topped the midweek chart, while Sean Paul trails at two with 'She Doesn't Mind'.
Labrinth enters at three with 'Last Time', while former number one song 'Somebody That I Used To Know' by Gotye slips to four.
JLS's Sport Relief single 'Proud' debuts at number five, Nicki Minaj slides to six with 'Starships' and Emeli Sandé is down to nine with 'Next To Me'.
Alexandra Burke tumbles eight places to 11 with 'Elephant', Jason Mraz continues to climb with 'I Won't Give Up' shifting up to 14 and You Me At Six land at 19 with new song 'The Swarm'.
Further down the chart, Rihanna's 'Talk That Talk' is down four to 33 and newcomer Angel begins to advance with 'Go In Go Hard' at 35.
Over on the album chart, Paul Weller is on course to achieve his fourth UK album chart-topper with new record Sonik Kicks.
The Military Wives are down to two with In My Dreams, David Guetta climbs to three with Nothing But The Beat and Emeli Sandé is at four with Our Version Of Events.
> View the current singles top 100 here
Listen to Katy Perry's 'Part of Me' below: