Tomb Raider clings on to second place in the UK charts.
Irrational Games' has landed in third position.
Defiance is a new IP designed to tie-in to the SyFy TV series of the same name, which premieres in the UK on April 16. The game sold the vast majority of its copies on Xbox 360, with 76 per cent of sales attributed to Microsoft's platform.
Square Enix's clings on to second place for the third week running, with EA's FIFA 13 also remaining in fourth.
3DS-exclusive Luigi's Mansion 2 remains fifth for its second week, while Epic Games' Gears of War: Judgement drops three places to sixth.
Call of Duty: Black Ops II is seventh, Assassin's Creed III eighth, Skylanders Giants ninth, and Need for Speed: Most Wanted is in tenth.
Monster Hunter 3 Ultimate on the 3DS and Wii U is nowhere to be seen in the full top 40, .
The Top 20 UK chart for the week ending April 6:
1. BioShock Infinite
2. Tomb Raider
3. Defiance
4. FIFA 13
5. Luigi's Mansion 2
6. Gears of War: Judgment
7. Call of Duty: Black Ops II
8. Assassin's Creed III
9. Skylanders Giants
10. Need for Speed: Most Wanted
11. Crysis 3
12. Far Cry 3
13. LEGO Batman 2: DC Super Heroes
14. Halo 4
15. Army of Two: The Devil's Cartel
16. LEGO The Lord of the Rings
17. Tiger Woods PGA Tour 14
18. SimCity
19. Sniper: Ghost Warrior 2
20. Sonic & All-Stars Racing Transformed