The track, produced by Max Martin and Dr Luke, marks her fifth consecutive chart-topper from her Teenage Dream album, matching the record set by Michael Jackson's 1987 album Bad.
The milestone comes as a result of combining sales of the original song and its remix, which features Missy Elliot and accounted for 25% of its downloads this week.
> Katy Perry Poll: What's your favourite Teenage Dream single?
Meanwhile, LMFAO dip to two with 'Party Rock Anthem' and Maroon 5 and Christina Aguilera's 'Moves Like Jagger' leaps from number eight to complete the top three.
Elsewhere in the top ten, Foster The People leap from 13 to eight with 'Pumped Up Kicks', and One Republic return to the edge of the top ten from 11 with 'Good Life'.
Further down the chart, Drake's 'Headlines' debuts at 13, Rihanna zooms from 50 to 25 with 'Cheers (Drink To That)' and Jason Derulo debuts at 39 with 'It Girl'.
David Guetta debuts at 66 with 'Titanium', Evanescence land at 68 with 'What You Want', and The Script land at 89 with 'Nothing'.
The Top 10 singles in full (click where possible for our reviews):
1. (2) Katy Perry: 'Last Friday Night (T.G.I.F)'
2. (1) LMFAO: 'Party Rock Anthem'
3. (8) Maroon 5 ft. Christina Aguilera: 'Moves Like Jagger'
4. (3) Nicki Minaj: 'Super Bass'
5. (5) Bad Meets Evil feat. Bruno Mars: 'Lighters'
6. (6) Lil Wayne: 'How To Love'
7. (4) Pitbull feat. Ne-Yo, Afrojack and Nayer: 'Give Me Everything'
8. (13) Foster The People: 'Pumped Up Kicks'
9. (9) Britney Spears: 'I Wanna Go'
10. (10) One Republic: 'Good Life'
Watch Katy Perry's 'Last Friday Night' music video below: