'Teenage Dream' repeats the success achieved by her new LP's lead single 'California Gurls' in June, knocking last week's number one from Taio Cruz into third spot.
X Factor runner-up Olly Murs scores his first top five hit with debut single 'Please Don't Let Me Go', while Pepper & Piano from Sky1's Must Be The Music enter the Top 10 at number nine.
Elsewhere in the chart, Adele's 'Make You Feel My Love' re-enters at number 22 after its use by an X Factor auditionee.
The Irish top ten singles in full:
1. (*) Katy Perry: 'Teenage Dream'
2. (2) Eminem ft. Rihanna: 'Love The Way You Lie'
3. (1) Taio Cruz: 'Dynamite'
4. (3) Flo Rida ft. David Guetta: 'Club Can't Handle Me'
5. (*) Olly Murs: 'Please Don't Let Me Go'
6. (5) Yolanda Be Cool & DCUP: 'We No Speak Americano'
7. (4) Brandon Flowers: 'Crossfire'
8. (6) Travie McCoy ft. Bruno Mars: 'Billionaire'
9. (*) Pepper & Piano: 'You Took My Heart'
10. (9) Swedish House Mafia: 'One (Your Name)'
> Click here to see this week's Irish album chart