The song, which is taken from the electro act's latest album Ocean Eyes, is followed by Lady GaGa's 'Bad Romance' in second place.
Haiti charity single 'Stranded', recorded by Jay-Z, Bono, The Edge and Rihanna, debuts in third position, followed by Iyaz's 'Replay' and OneRepublic's 'All The Right Moves' in fourth and fifth respectively.
Meanwhile, Australian band The Temper Trap climb up ten spots to number eight with 'Sweet Disposition'.
The top ten singles in full (click where possible for our reviews):
1. (1) Owl City: 'Fireflies'
2. (3) Lady GaGa: 'Bad Romance'
3. (-) Jay-Z, Bono, The Edge & Rihanna: 'Stranded (Haiti Mon Amour)'
4. (2) Iyaz: 'Replay'
5. (9) OneRepublic: 'All The Right Moves'
6. (6) Glee Cast: 'Don't Stop Believin'
7. (4) Sidney Samson ft. Wizard Sleeve: 'Riverside (Let's Go!)'
8. (18) The Temper Trap: 'Sweet Disposition'
9. (7) 3OH!3 ft. Katy Perry: 'Starstrukk'
10. (10) The Saturdays: 'Ego'