The track scores a second week at the summit, holding off competition from Wynter Gordon's 'Dirty Talk', which remains at two.
Meanwhile, the Black Eyed Peas vault from 19 to three with 'Just Can't Get Enough', and Usher's 'More' leaps from 13 to seven.
Other chart climbers were Far*East Movement and Ryan Tedder's 'Rocketeer', which advances from 24 to 18, while Lupe Fiasco's 'The Show Goes On' leaps from 45 to 32.
Further down the chart, Dr Dre's 'I Need A Doctor' debuts at 43, and Birds Of Tokyo's 'Wild At Heart' lands at 50.
The top ten singles in full (click where possible for our reviews):
1. (1) Rihanna: 'S&M'
2. (2) Wynter Gordon: 'Dirty Talk'
3. (19) Black Eyed Peas: 'Just Can't Get Enough'
4. (3) Enrique Iglesias ft. Ludacris & DJ Frank E: 'Tonight (I'm Lovin' You)'
5. (4) Alexis Jordan: 'Happiness'
6. (5) Katy Perry: 'E.T'
7. (13) Usher: 'More'
8. (8) Avril Lavigne: 'What The Hell'
9. (6) Chris Brown: 'Yeah 3X'
10. (9) David Guetta ft. Rihanna: 'Who's That Chick?'