The track - which features an appearance from rapper Snoop Dogg - also leads the US, Australian and Irish singles chart this week.
It becomes her second single to reach No.1 in the UK after 2008's 'I Kissed A Girl'.
K'Naan's 'Wavin' Flag' holds steady at two, while last week's chart-topper, 'Shout' by Shout For England, slips to three.
Meanwhile, Eminem places two singles in this week's top ten: 'Not Afraid' at five and 'Love The Way You Lie', featuring Rihanna, at seven.
Lady GaGa's 'Alejandro' continues its climb up the chart, advancing from ten to eight.
Outside of the top ten, Scissor Sisters debut at eleven with 'Fire With Fire' to notch up their eighth top 20 hit, while Kelly Rowland climbs from 17 to 13 with 'Commander'.
The top ten singles in full:
1. (-) Katy Perry ft. Snoop Dogg: 'California Gurls'
2. (2) K'Naan: 'Wavin' Flag'
3. (1) Shout For England ft. Dizzee Rascal & James Corden: 'Shout'
4. (3) Example: 'Kickstarts'
5. (6) Eminem: 'Not Afraid'
6. (4) Kylie Minogue: 'All The Lovers'
7. (-) Eminem & Rihanna: 'Love The Way You Lie'
8. (10) Lady GaGa: 'Alejandro'
9. (5) Tinie Tempah ft. Labrinth: 'Frisky'
10. (7) David Guetta & Chris Willis ft. Fergie & LMFAO: 'Gettin' Over You'
Source: Official Charts Company
> Click here for this week's top ten albums