The track - which features soul singer Janelle Monae - bounds from six to one on the tally, followed by Gym Class Heroes at two with 'Ass Back Home' and Flo Rida's 'Wild Ones' at three.
Meanwhile, Nicki Minaj's 'Starships' leaps from ten to four, One Direction rises from nine to eight with 'One Thing', and Skirllex's 'Bangarang' advances from 12 to nine.
Further down the chart, Taio Cruz's 'Troublemaker' vaults from 38 to 11, Train jump from 20 to 16 with 'Drive By', and Far*East Movement and Justin Bieber's 'Live My Life' shifts from 42 to 29.
Knife Party climb from 50 to 32 with '100% No Modern Talking', and Carly Rae Jepsen debuts at 39 with 'Call Me Maybe'.
The Top 10 singles in full (click where possible for our reviews):
1. (6) fun. ft. Janelle Monae: 'We Are Young'
2. (1) Gym Class Heroes ft. Neon Hitch: 'Ass Back Home'
3. (2) Flo Rida ft. Sia: 'Wild Ones'
4. (10) Nicki Minaj: 'Starships'
5. (4) 360 ft. Gossling: 'Boys Like You'
6. (3) Qwote ft. Pitbull: 'Throw Your Hands Up'
7. (5) Jason Derulo: 'Fight For You'
8. (9) One Direction: 'One Thing'
9. (12) Skirllex: 'Bangarang'
10. (7) Chris Brown: 'Turn Up The Music'
Watch fun.'s 'We Are Young' music video below: