The R&B singer ends Iyaz's two-week reign in peak position with 'Replay', which slips one place to two.
Taylor Swift rises three places with 'Today Was A Fairytale', while Owl City's 'Fireflies' drops two places to four.
British group The Midnight Beast leap eleven places to five with their YouTube cover-parody of 'Tik Tok' and Ke$ha's original version drops six places to 11.
Rihanna climbs nine places to eight with 'Rude Boy', while Cheryl Cole enjoys her first ever Top Ten success in Australia with will.i.am collaboration '3 Words'.
Elsewhere, Haiti charity song 'We Are The World 25 For Haiti' is the second-highest new entry at 18. Simon Cowell's all-star charity single 'Everybody Hurts' enters at 28.
The top ten singles in full (click where possible for our reviews):
1. (-) Jason Derulo: 'In My Head'
2. (1) Iyaz: 'Replay'
3. (6) Taylor Swift: 'Today Was A Fairytale'
4. (2) Owl City: 'Fireflies'
5. (16) The Midnight Beast: 'Tik Tok (Parody)'
6. (4) David Guetta ft. Kid Cudi: 'Memories'
7. (3) Ke$ha ft. 3OH!3: 'Blah Blah Blah'
8. (17) Rihanna: 'Rude Boy'
9. (25) Cheryl Cole: '3 Words'
10. (7) Jay Sean ft. Sean Paul, Lil Jon: 'Do You Remember?'