The 'Billionaire' collaborator's track, which first topped the chart four weeks ago, jumped past previous number one Cee Lo Green's 'Forget You' after it was covered by X Factor favourite Matt Cardle on last week's show.
Mars's success sees The Wanted settle for second place with their latest single 'Heart Vacancy', while other new entries - Katy Perry's 'Firework', Jay Sean's '2012' and 'Best Behaviour' by N-Dubz - also chart highly, landing at five, nine and ten respectively.
Elsewhere on the chart, two more new arrivals, Nelly's 'Just A Dream' and Diana Vickers's 'My Wicked Heart', just fail to break the top ten landing at 11 and 13.
Further down, Adele makes a slight gain with 'Make You Feel My Love', which climbs from 20 to 17, and Robbie Williams's long-awaited reunion track with Gary Barlow, 'Shame', tumbles 13 places to 20.
Meanwhile, the X Factor-related success present at the top of the chart fails to transfer to last year's champ Joe McElderry, as 'Ambitions' slips out of the top 20 in its second week of release.
The top ten in full:
1. (2) Bruno Mars: 'Just The Way You Are'
2. (-) The Wanted: 'Heart Vacancy'
3. (3) Duck Sauce: 'Barbra Streisand'
4. (1) Cee Lo Green: 'Forget You'
5. (-) Katy Perry: 'Firework'
6. (9) Mike Posner: 'Cooler Than Me'
7. (4) Swedish House Mafia Vs Tinie Tempah: 'Miami 2 Ibiza'
8. (8) Tinie Tempah: 'Written In The Stars'
9. (-) Jay Sean ft. Nicki Minaj: '2012 (It Ain't The End)'
10. (-) N-Dubz: 'Best Behaviour'
Source: Official Charts Company
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