The Brazilian's transfer fee was a mere £8 million and he could prove to be quite the bargain for Unai Emery.
The Gunners were desperate to improve their backline this summer and with the arrival of Luiz - and Kieran Tierney - they've done just that.
It looked like Chelsea were missing him on Sunday too, as they were battered 4-0 by Manchester United at Old Trafford.
So why did Frank Lampard let him leave? Well, the Blues boss revealed that 'honest conversations' between the two parties led to the 32-year-old's departure.
- Ozil and Kolasinac could miss more games
- Twitter thread explains Arsenal's great summer
- Emery's brilliant reaction to Joe Willock tackle
Now he's secured his move, Luiz has outlined publically why he left Stamford Bridge - claiming that he's made the switch to win trophies.
"All my life, I've always started the season with the goal to win things," he told Arsenal's website.
"It's not going to be different here -- I'm going to come with this mentality because this is what made me move here."
You've got to admire his aspirations, but they doesn't exactly add up, given the contrasting fortunes of Chelsea and Arsenal over the last decade.
In the last 10 years, the Gunners have won just three major trophies - all of them being the FA Cup.
By contrast, Chelsea have won 10 major honours - three Premier League titles, three FA Cups, one League Cup, one Champions League and two Europa Leagues.
So, if Luiz really wanted to win trophies, perhaps he should have stayed in west London.
After all, it looks like Lampard's side really could have done with his cover on Sunday.
Kurt Zouma and Andreas Christensen just didn't look confident enough against Man United and if there's one man who could install a bit more of that, it's Luiz.
But he's made his choice, and we'll have to see if he'll be able to achieve his goal of winning more trophies at Arsenal.News Now - Sport News