The singer's track 'Grenade' ends Matt Cardle's four-week stay at the top spot and repeats the success of his debut release 'Just The Way You Are' last year. Cardle slips to number two, while Rihanna holds strong at number three with 'What's My Name?'.
Britney Spears makes a top 10 debut at number six with 'Hold It Against Me', the lead single from her highly-anticipated seventh LP.
Meanwhile, Adele's 'Make You Feel My Love' climbs 10 spots to number 12, while Tinie Tempah enjoys a five-place jump to number 13 with 'Invincible'. Brit Awards Critics' Choice winner Jessie J debuts at number 17 with her number two UK hit 'Do It Like A Dude'.
Elsewhere in the chart, new entries come from Usher, Cee Lo Green and Ryan Sheridan.
This week's top ten in full:
1. (*) Bruno Mars: 'Grenade'
2. (1) Matt Cardle: 'When We Collide'
3. (3) Rihanna ft. Drake: 'What's My Name?'
4. (2) Rubberbandits: 'Horse Outside'
5. (8) Rihanna ft. David Guetta: 'Who's That Chick'
6. (*) Britney Spears: 'Hold It Against Me'
7. (4) Black Eyed Peas: 'The Time (Dirty Bit)'
8. (5) Katy Perry: 'Firework'
9. (7) Ellie Golding: 'Your Song'
10. (10) Kesha: 'We R Who We R'