'Just The Way You Are' - currently number one in the UK and Ireland as well - jumps from three to the summit this week.
Mars, a 25-year-old singer-songwriter from Honolulu, Hawaii, previously featured on hits by B.o.B. ('Nothin' On You') and Travie McCoy ('Billionaire').
Rihanna holds steady at two with former chart-topper 'Only Girl (In The World)', while Nelly climbs from four to three with 'Just A Dream'.
Last week's number one, 'Raise Your Glass' by Pink, slips to four.
Meanwhile, Aussie pop singer Zoe Badwi notches up her first top ten hit this week, vaulting from 18 to nine with 'Freefallin'.
Pitbull and T-Pain also enter the top ten with 'Hey Baby (Drop It The To The Floor)', which climbs from 16 to a new peak of ten.
The top ten singles in full (click where possible for our reviews):
1. (3) Bruno Mars: 'Just The Way You Are'
2. (2) Rihanna: 'Only Girl (In The World)'
3. (4) Nelly: 'Just A Dream'
4. (1) Pink: 'Raise Your Glass'
5. (5) Mike Posner: 'Cooler Than Me'
6. (6) Taio Cruz: 'Dynamite'
7. (7) Cee Lo Green: 'F**k You'
8. (8) B.o.B. ft. Rivers Cuomo: 'Magic'
9. (18) Zoe Badwi: 'Freefallin'
10. (16) Pitbull ft. T-Pain: 'Hey Baby (Drop It To The Floor)'