The Barbadian singer originally arrived in the runner-up spot behind Cheryl Cole's 'Promise This' last Sunday, but reached the summit following improved second-week sales of more than 134,000 copies.
In notching up her fourth UK number one, Rihanna sends X Factor judge Cole falling to two this week, just ahead of 'Happiness', the debut single from America's Got Talent semi-finalist Alexis Jordan.
A few climbers pepper the upper section of the chart as well, with Katy Perry's 'Firework' advancing two places to four, while The Saturdays achieve their eight UK top ten hit, after their Flo Rida collaboration 'Higher' rises to ten from last week's 22.
Looking just beyond the top of the chart, will.i.am and Nicki Minaj arrive at 11 with new release 'Check It Out', while 1992 classic 'Stay' by Shakespears Sister re-enters at 12 following Cher Lloyd's "performance of the season" on last week's X Factor.
Meanwhile, two former chart-topping acts make somewhat disappointing returns, as Cheryl Cole's Girls Aloud colleague Nadine Coyle fails to replicate her bandmate's success with 'Insatiable', which arrives at 26, and Roll Deep follow two successive number one singles with a new entry at 29 in the shape of 'Take Control'.
The top ten in full:
1. (2) Rihanna: 'Only Girl (In The World)'
2. (1) Cheryl Cole: 'Promise This'
3. (-) Alexis Jordan: 'Happiness'
4. (6) Katy Perry: 'Firework'
5. (4) Cee Lo Green: 'Forget You'
6. (3) Bruno Mars: 'Just The Way You Are'
7. (5) Mike Posner: 'Cooler Than Me'
8. (8) Nelly: 'Just A Dream'
9. (7) Duck Sauce: 'Barbra Streisand'
10. (22) The Saturdays ft. Flo Rida: 'Higher'
Source: Official Charts Company
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